This one is going to be a painful one. As BC is a bit of a corner case, 1.6.4 worlds do not properly load in 1.7.2. This seems to be due to the "submod" characteristic of BC. According to @cpw, the way to make progress on this is:

1. release a 1.6.4-based version of BC registering all blocks under this same mod id
2. have people load and save their world with that version
3. have the 1.7.2 version(s) of BC following the same scheme

Investigations ended up to some potential forge-related issues, MinecraftForge/MinecraftForge#1015.

Probably originally linked to MinecraftForge/MinecraftForge#996 actually. 1.6.4 migration work will restart after that.

Forge issue MinecraftForge/MinecraftForge#996 is now fixed.

Just tried this on a 1.6.4 world last night with Buildcraft 5.0.6, appears fixed for me.

I think this can be closed.