AT&T in November of last year began enabling FaceTime over cellular for iPhones running iOS 6 that had the ability to connect to 4G LTE, in other words, the iPhone 5. It was supposed to be limited to tiered data plans and shared data plans only.
Monday AT&T has revealed to The Verge that they will be enabling FaceTime over cellular for everyone and anyone, as long as they have an iOS 6 device. The new system will allow all subscribers with iOS 6 devices (because Apple enabled FaceTime over cellular in iOS 6 only) to use FaceTime over cellular whether they’re on 4G LTE, 4G HSPA+, or even 3G. It also will not matter what kind of data plan the subscriber is paying for, whether it's capped or unlimited:
Originally Posted by AT&T
Once FaceTime over cellular is enabled for all customers, on all data plan types and cellular speeds, not only will iPhones be able to use the feature over a cellular network, but so will iPads.
Notably, while everyone is still waiting to use FaceTime, users can still download non-pre-loaded video chat applications from the App Store to use, such as Skype, Tango, and others to video chat over their cellular network.
Sources: The Verge