In shote span of time of released iOS 4 jailbreak, a new software is devised to make Facetime work over a 3G connection. As of now, Facetime only officially works over a WiFi connection, and is cited as a huge limitation of the popular video chatting service.
"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. 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."
Facetime over 3G Video: