A lot of people out there may have been wondering if AT&T had come up with some scheme that would have allowed it to count Face Time video calls against their calling minutes. According to Apple, Face Time calls will not use voice minutes.
If you start a Face Time call from within a voice call, the voice call is terminated when Face Time connects. That means that you will not use your voice minutes. With AT&T not charging between AT&T subscribers, this was a bit of a moot point in the US.
However, Apple is said to be working on Face Time for 3G calling in the next update. If that happens, I can see AT&T charging you for voice minutes as well as data on the same call.