Back in September of 2012, Apple made FaceTime available over cellular to everyone, but AT&T made the service available only to those with LTE data on a tiered or shared plan. AT&T went ahead and blocked its service to users with only 3G data subscriptions.
Now, AT&T announced, "We will be enabling FaceTime over Cellular at no extra charge for customers with any tiered data plan using a compatible iOS device." AT&T is currently in the process of updating those plans so their users can start taking advantage of FaceTime. Tough luck If you still have a "grandfathered" plan. You will not be able to use the new Facetime service. AT&T will be rolling out the update automatically over the next few weeks.