So are NIB files still binary, or have they changed to text-based so they can be version controlled better?

I'm not sure that sort of stuff is public knowledge.

Nib files can already be text in Tiger -- you just have to explicitly save them as such. It doesn't help a heck of a lot, because any single change appears to cause all of the object IDs in the file to change. The non-public question is whether or not Leopard fixes that problem.