I hate having to mention this every time Google Voice comes up, but if you are using Google Voice from your AT&T phone, it uses your AT&T minutes. Just like any other call. (It is not VoIP until your voice gets to Google's servers. Then it travels over IP, until it is converted to POTS at the other end.)

The only loss for AT&T is that SMS messages use your data plan, and that you can call people (and receive calls from them) from a phone other than your cell phone.

Personally, I think killing this off was really stupid. It caused them lots of regulatory scrutiny for something that only a few users use. (GV is still invite-only, after all.)