As expected, AT&T has officially announced it's limiting how iPhone users can use FaceTime over 3G/4G when Apple's new iOS 6 software launches.
You won't be able to use FaceTime over 3G unless you sign up for one of its data sharing plans that lets you buy data and share it across all your connected devices.
If you have an individual data plan with AT&T, you will only be able to use FaceTime over Wi-Fi.
Sprint has already committed to letting users use FaceTime over a cellular connection regardless of the plan customers choose.
Verizon has not said how it'll handle FaceTime over 3G yet.
Here's the statement from AT&T:
AT&T will offer FaceTime over Cellular as an added benefit of our new Mobile Share data plans, which were created to meet customers’ growing data needs at a great value. With Mobile Share, the more data you use, the more you save. FaceTime will continue to be available over Wi-Fi for all our customers.