I got a resource pack (this was the same with texture packs in 1.5.2), but when I switched everything was the same but when I loaded my testing world, I only saw the void. Switching back to the default resource pack didn't help.

How to reproduce:

1. Change level.dat->Data->Player->abilities->walkSpeed to 0 (it doesn't seem to work without doing this)
2. Change to another resource pack.
3. Load up a new world, and you can't see any blocks!
Also, you have to make a new minecraft.jar to fix this, as far as I know.

EDIT:
It seems to do this in 1.6.2 without changing resource packs.

EDIT 2:
It seems to be fixed in 13w36b.