AT&T has allowed users with both tiered and unlimited users to have FaceTime on LTE. Restart your iPhone to have the option enabled.
AT&T is famous for annoying their customers with iPhones. At the launch of iOS 6 AT&T made it so only members with a shared data plan would be able to use FaceTime off 4G. It seems AT&T has learned that was a reason many people were leaving their network for Verizon and will make it so anyone without an unlimited data plan can use FaceTime over cellular.
Of course it says LTE device meaning iPad 3,4 and iPhone 5 customers exclusively with the upcoming LTE iPad mini as well. It says LTE models but that doesn’t necessarily mean that it will not run off of AT&T 4G hspa+ networks as LTE doesn’t cover very many places. This will be available in the next 8 to 10
This means users with an LTE enabled device running on AT&T will be able to FaceTime on cellular data as long as you don’t have an unlimited data plan.