2/3 days ago it came out that AT&T has enabled FaceTime over cellular for some iPhone/iPad cellular plans. If you've tested this and it works for you, could you please post your iDevice model and baseband?
I've checked the following:
White 32 GB AT&T iPhone 5 (MD637LL) running iOS 6.0 on 1.00.16 baseband:
Settings > General > Cellular > FaceTime appears, but message pops up to call AT&T to enable.
Black 16 GB AT&T iPhone 4 (MC318LL) running iOS 6.0 (jailbroken) on 4.12.02 baseband:
Settings > General > Cellular > FaceTime does not appear.