There is a little bug while spawning spikes. They all look like this:

When i click on the Snow under the spikes, it disappears and looks like this:

I haven't changed anything in the "Ice Plaines Spikes" biome configuration.

IceSpike(PACKED_ICE,HugeSpike,3,1.66,60,128,ICE,DIRT,SNOW_BLOCK)
IceSpike(PACKED_ICE,SmallSpike,3,98.33,60,128,ICE,DIRT,SNOW_BLOCK)
IceSpike(PACKED_ICE,Basement,2,100.0,60,128,ICE,DIRT,SNOW_BLOCK)

Ok, i think the problem isn't caused by terrain control. I created the world using "World Painter" (with the option to let minecraft populate the terrain - everything else works fine) and it doesen't matter if i use the MC Singleplayer, bukkit or terrain control, the spikes will always float. Confusing...

Looks fine for me. Can I have the lines you're using to spawn the spikes?

Problem solved. Previously i had snow blocks with a snow layer (ID 78) on the top. This replaced air over the snow and the spikes spawn on top of block 78. When you use the block with the ID 80 on the top, this effect dissapears and everythings looks fine. Maybe it is possible to make terrain control replaces the block ID 78, insteed of spawning it one block higher ;)

Yes, I think I can add that.

@kneepkiller Found out that you can just add SNOW to the source blocks.

This will generate the spikes correctly when using WorldPainter:

IceSpike(PACKED_ICE,HugeSpike,3,1.66,60,128,ICE,DIRT,SNOW_BLOCK,SNOW)
IceSpike(PACKED_ICE,SmallSpike,3,98.33,60,128,ICE,DIRT,SNOW_BLOCK,SNOW)
IceSpike(PACKED_ICE,Basement,2,100.0,60,128,ICE,DIRT,SNOW_BLOCK,SNOW)