They market it as facetime over CELLULAR not 3G.
FaceTime now works over cellular networks as well as Wi-Fi, so you can make and receive FaceTime calls wherever you happen to be.3 You can even make and receive FaceTime calls on your iPad using your phone number. That means you can use FaceTime wherever you are, on any device. And never miss another wink, smile, air kiss, or eye roll.
Read carefully. And in this case, it's not just the regular 3G, but only the cellular technology used by 4S onwards