I've the latest of both inventory-tweaks and Forge. Other mods load fine, but if I have invtweaks set to load, the client freezes up when I try to load a single player world.

2013-03-27 20:07:41 [FINEST] [mcp] Sending event FMLServerAboutToStartEvent to mod mcp
2013-03-27 20:07:41 [FINEST] [mcp] Sent event FMLServerAboutToStartEvent to mod mcp
2013-03-27 20:07:41 [FINEST] [FML] Sending event FMLServerAboutToStartEvent to mod FML
2013-03-27 20:07:41 [FINEST] [FML] Sent event FMLServerAboutToStartEvent to mod FML
2013-03-27 20:07:41 [FINEST] [inventorytweaks] Sending event FMLServerAboutToStartEvent to mod inventorytweaks
2013-03-27 20:07:41 [FINEST] [inventorytweaks] Sent event FMLServerAboutToStartEvent to mod inventorytweaks
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] Fatal errors were detected during the transition from LOADING to SERVER_ABOUT_TO_START. Loading cannot continue
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader]
mcp Minecraft Coder Pack Unloaded->Available
FML Forge Mod Loader Unloaded->Available
inventorytweaks Inventory Tweaks Unloaded->Errored
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] The following problems were captured during this phase
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] Caught exception from inventorytweaks
java.lang.NullPointerException
at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:512)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:165)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:98)
at cpw.mods.fml.common.Loader.serverAboutToStart(Loader.java:813)
at cpw.mods.fml.common.FMLCommonHandler.handleServerAboutToStart(FMLCommonHandler.java:311)
at net.minecraft.server.integrated.IntegratedServer.func_71197_b(IntegratedServer.java:115)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:355)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] A fatal exception occurred during the server about to start event
java.lang.NullPointerException
at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:512)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:165)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:98)
at cpw.mods.fml.common.Loader.serverAboutToStart(Loader.java:813)
at cpw.mods.fml.common.FMLCommonHandler.handleServerAboutToStart(FMLCommonHandler.java:311)
at net.minecraft.server.integrated.IntegratedServer.func_71197_b(IntegratedServer.java:115)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:355)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
2013-03-27 20:07:41 [INFO] [Minecraft-Server] Stopping server
2013-03-27 20:07:41 [INFO] [Minecraft-Server] Saving players
2013-03-27 20:07:41 [INFO] [Minecraft-Server] Saving worlds
2013-03-27 20:07:41 [INFO] [STDERR] java.lang.NullPointerException
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.func_71267_a(MinecraftServer.java:271)
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.func_71260_j(MinecraftServer.java:316)
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.integrated.IntegratedServer.func_71260_j(IntegratedServer.java:224)
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:447)
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
2013-03-27 20:07:41 [FINEST] [mcp] Sending event FMLServerStoppedEvent to mod mcp
2013-03-27 20:07:41 [FINEST] [mcp] Sent event FMLServerStoppedEvent to mod mcp
2013-03-27 20:07:41 [FINEST] [FML] Sending event FMLServerStoppedEvent to mod FML
2013-03-27 20:07:41 [FINEST] [FML] Sent event FMLServerStoppedEvent to mod FML
2013-03-27 20:07:41 [FINEST] [inventorytweaks] Sending event FMLServerStoppedEvent to mod inventorytweaks
2013-03-27 20:07:41 [FINEST] [inventorytweaks] Sent event FMLServerStoppedEvent to mod inventorytweaks
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] Fatal errors were detected during the transition from ERRORED to SERVER_STOPPED. Loading cannot continue
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader]
mcp Minecraft Coder Pack Unloaded->Available->Available
FML Forge Mod Loader Unloaded->Available->Available
inventorytweaks Inventory Tweaks Unloaded->Errored->Errored
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] The following problems were captured during this phase
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] Caught exception from inventorytweaks
java.lang.NullPointerException
at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:512)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:165)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:98)
at cpw.mods.fml.common.Loader.serverAboutToStart(Loader.java:813)
at cpw.mods.fml.common.FMLCommonHandler.handleServerAboutToStart(FMLCommonHandler.java:311)
at net.minecraft.server.integrated.IntegratedServer.func_71197_b(IntegratedServer.java:115)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:355)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
2013-03-27 20:07:41 [SEVERE] [ForgeModLoader] Caught exception from inventorytweaks
java.lang.NullPointerException
at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:512)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:165)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
at com.google.common.eventbus.EventBus.post(EventBus.java:267)
at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:98)
at cpw.mods.fml.common.Loader.serverStopped(Loader.java:796)
at cpw.mods.fml.common.FMLCommonHandler.handleServerStopped(FMLCommonHandler.java:470)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:456)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
2013-03-27 20:07:41 [INFO] [STDERR] Exception in thread "Server thread" java.lang.NullPointerException
2013-03-27 20:07:41 [INFO] [STDERR] at cpw.mods.fml.common.FMLModContainer.handleModStateEvent(FMLModContainer.java:512)
2013-03-27 20:07:41 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
2013-03-27 20:07:41 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
2013-03-27 20:07:41 [INFO] [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
2013-03-27 20:07:41 [INFO] [STDERR] at java.lang.reflect.Method.invoke(Unknown Source)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventBus.post(EventBus.java:267)
2013-03-27 20:07:41 [INFO] [STDERR] at cpw.mods.fml.common.LoadController.propogateStateMessage(LoadController.java:165)
2013-03-27 20:07:41 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
2013-03-27 20:07:41 [INFO] [STDERR] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
2013-03-27 20:07:41 [INFO] [STDERR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
2013-03-27 20:07:41 [INFO] [STDERR] at java.lang.reflect.Method.invoke(Unknown Source)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventHandler.handleEvent(EventHandler.java:74)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.SynchronizedEventHandler.handleEvent(SynchronizedEventHandler.java:45)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatch(EventBus.java:314)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventBus.dispatchQueuedEvents(EventBus.java:296)
2013-03-27 20:07:41 [INFO] [STDERR] at com.google.common.eventbus.EventBus.post(EventBus.java:267)
2013-03-27 20:07:41 [INFO] [STDERR] at cpw.mods.fml.common.LoadController.distributeStateMessage(LoadController.java:98)
2013-03-27 20:07:41 [INFO] [STDERR] at cpw.mods.fml.common.Loader.serverAboutToStart(Loader.java:813)
2013-03-27 20:07:41 [INFO] [STDERR] at cpw.mods.fml.common.FMLCommonHandler.handleServerAboutToStart(FMLCommonHandler.java:311)
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.integrated.IntegratedServer.func_71197_b(IntegratedServer.java:115)
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:355)
2013-03-27 20:07:41 [INFO] [STDERR] at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)

Install Forge (http://files.minecraftforge.net/minecraftforge/minecraftforge-universal-recommended.zip), not the FML stand-alone package.

For the record I've used Modloader untill about 4 hours ago =3