Based on the choppiness of the FaceTime video, it appears to me that this is the reason at&t does not allow this to happen right now. It looks like the 3G network will work great with Data however, not with Video yet. Please read the entire blog first.
At the current time, the only way to be able to use FaceTime with the iPhone 4 without Wi-Fi is by Jailbreaking it. Now before I get any comments about this information, I would like to go on record as saying that I do not condone or recommend this practice. That being said, I have also Jailbroken my original iPod after the warranty ran out years ago and then I sold it and have upgraded my iPhones just as the previous warranties have run out also so, I haven't felt the need to do this again in the last 3 years. However, it is available for those of you that just have to do this sort of thing for whatever reason you feel is justified. Do this at your own risk.
Here is the link to the process and be sure and watch the video before deciding to do this: http://www.youtube.com/watch?v=_Kq7Ckcgp0o&feature=player_embedded