There's still the problem that most people don't have unlimited data on their iPhone 4 and so won't be able to make too much use of FaceTime over 3G. The networks saw this coming, I'd think.
When it does launch on 3G, it'll probably not come out of your data allowance and they'll bill it separately. Kind of like how you get included minutes and texts, you'll get FaceTime minutes. Just my prediction.