FaceTime over AT&T's 4G (you know, 3G) on my iPhone 4s (with a 2GB data plan) was enabled as of a few days ago. Haven't checked the settings since then, but I just updated to iOS 6.1 from 6.0.1, and after a message came up about saying that the iPhone was now using the new iCloud email address for iMessages, I decided to check on FaceTime, and it appears that the cellular data option for FaceTime is switched off again (and I can't enable it, just shows the 'call AT&T' popup). I don't really know for sure if I can blame iOS 6.1 because as its been a few days since I checked the setting it could be coincidence.
What do you think? A glitch in AT&T's system that accidentally temporarily enabled my 4G FaceTime? Something about the update turning it off? When I asked AT&T, they were surprised that it was even working at all a few days ago.