Initially Apple's FaceTime, the built-in app that allows you to video conference with other iOS device users, only worked over WiFi. Then with the release of iOS 6 last fall, Apple made it possible to use FaceTime via the cellular data network. However, it was up to the specific carrier to enable this capability on its network. And AT&T held back, limiting it to users who were on a specific plan. But, according to AppleInsider, multiple reports say FaceTime is being enabled in particular areas, strongly suggesting that AT&T is now rolling out this capability nationwide, including on both LTE and HSPA+ networks.
There hasn't been an official announcement other than one earlier this year in which AT&T promised roll it out only to those on its LTE network in June.
Of course, FaceTime's live streaming video can use up quite a bit of data, so unless you're on an unlimited data plan, you'll need to be cautious about using this feature.