As noted by AppleInsider, AT&T has begun its FaceTime over cellular rollout to users. Back in May, AT&T stated that, by the end of 2013, all users would be able to use FaceTime over cellular. It appears that transition has begun. According to AppleInsider:
So far, activation of the service has been verified in parts of New York, Maryland, Georgia, Louisiana, California and Hawaii. Voice and video quality is nearly on a par with the WiFi implementation of the service, suggesting AT&T is not throttling those subscribers who choose to use the feature while on the go.
In January AT&T began rolling out FaceTime over cellular to users of its tiered data plans. By the time this current rollout is complete, all AT&T users will be able to use FaceTime over cellular, including those on grandfathered-in unlimited data plans.
[Editor's Note: A couple of weeks ago I noticed FaceTime over cellular was enabled for me, and I'm in Tennessee -- others confirm similar activations in the state. - VA]