FaceBreak is a tweak available in Cydia (which just went back up, although some “connection timeout” issues are still there) at $1.49 that allows you to have FaceTime calls on 3G. It’s very simple and focused on FaceTime, unlike other apps such as My3G or 3G Unrestrictor that let you modify other settings and options for other 3rd party apps installed on your device.
The latest 1.1.5 update brings iOS 4.1 support, bug fixes, better battery handling. Available through BigBoss’ repo at $1.49. Also, check out our continually updating list of Cydia apps working on iOS 4.1 here.