We attempted to bring up the same same error message when enabling the FaceTime over Cellular feature on a Verizon iPad WiFi + Cellular, but the feature enabled without an issue. AT&T charges for a separate plan to use Personal Hotspot on its network, and the carrier appears set to replicate that process with FaceTime over Cellular.
AT&T provided the following official statement on the matter:
"We're working closely with Apple on the new developer build of iOS6 and we'll share more information with our customers as it becomes available."
To replicate the message in iOS 6 beta 3 you need to restore your network settings.