FaceTime had always been restricted to wifi on the 4, no differnt with FaceTime audio I believe.
It would make more sense to only restrict FaceTime video to wifi only. I assume the restriction is due to the low data speed on the 4 and 4s. It makes no sense to restrict FaceTime audio, however. I can't believe the data speed couldn't handle it.
It's not just about the data speed, the 3G module in your phone is only HSDPA 7.2mbps which have higher ping times (90-100ms lowest) thus making VoIP calls have delay, I think apple doesn't want that. Have you tried skyping over 3G? how is it? Just for comparison's sake.
HSPA+ have better ping times 50ms-100ms
Yes, I've used Skype (audio only) on 3G, and it's perfectly fine. I don't understand why Apple won't allow it with FaceTime.
AT&T recently lifted the restriction (even on unlimited data users) to facetime over 3G/4G.
Which carrier are you on?
That's not true. I still can't facetime over the 3G network on my iPhone 4 with a grandfathered unlimited data plan on AT&T.