Initial developer releases of iOS 5 have FaceTime over 3G enabled but in final iOS 5 that feature was disabled by default, so just by following the tweak below you can enable FaceTime over 3G in iOS 5.
Enable Native FaceTime over 3G on iOS 5
- Your iDevice must run over jailbroken iOS 5
- 3G data access
Navigate to “/System/Library/CoreServices/SpringBoard.app/” through file browser
Open “N90AP.plist” file and under following lines of code
Done! enjoy FaceTime on jailbroken iOS 5 over 3G without installing any third party application.