Preview at WWDC likely in June, followed by September launch.
Verizon to Support FaceTime over Cellular on iPhone 5, iPhone 4S and New iPad
Verizon told MacRumors today that "if FaceTime works via cellular on any Apple device, it will work on any Verizon Wireless data plan with no special arrangements."
AT&T said something similar, confirming today that FaceTime over Cellular would work on all supported devices, so long as the customer has a new Mobile Share data plan. Sprint has said in the past that it is committed to unlimited data and does not discriminate based on the application used.
Apple's iOS 6 footnotes say that the only supported devices for FaceTime over Cellular are the iPhone 4S, iPhone 5, or iPad (3rd generation) with cellular data capability.