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If you've jailbroken your iOS device and don't want to go through the hassle of manipulating the system files to make FaceTime calls over 3G then checkout jailbreak tweak FaceBreak.
Yaniv Dunan - developer of FaceBreak has been recently updated it to support iOS 5/iOS 5.0.1.
Here's a quick description of the jailbreak tweak on Cydia: