ATT vs Verizon iPhone batteries

I was just reading the iphone 4 Verizon tear down, and while most things are not only compared, they are also listed as interchangeable, or not interchangeable (the vast majority being the latter). One thing was missing. While the new Verizon battery was shown to have a new model number (and it was also mentioned that the new battery weighted slightly less), there was no mention of whether or not the two batteries were interchangeable...i.e. could you put a Verizon battery into an ATT iphone?

Of course, this leads to my second question (and I'm sorry if this one has been answered already)...are these batteries available through a third party (including ifixit?)

Thanks very much...love the site

Todd