For jailbreakers, at least. My3G, the tweak that makes iOS believe it's on Wifi when it's actually using 3G, has been updated to version 4.1 to include support for FaceTime. Simple as that, you just have to check an option.
My3G is available on Rock ($3.99) or through ModMyi's repo in Cydia at $2.79. I haven't tested the app yet, but I'll keep you posted about my results. It'll also be interesting to hear from American users whether AT&T's network will be able to undergo the traffic of jailbreakers or not. Please let us know in the comments below.
This is huge, anyway.
UPDATE: ModMyi's repo seems to be down at the moment. It should be back online in a hour or so.
UPDATE #2: Check out the video of FaceTime with My3G below, courtesy of 9to5mac.