If you remember, Apple sent the iPhone 4 without enabling facetime in some countries esp in Middle East. But fortunately you can remove these restrictions / limitations after installing a simple tweak, MobileWhat, available for jailbreak iPhone 4 from Cydia Store.
Now the team behind MobileWhat is working on a new tweak called FaceIt-3GS that will enable FaceTime on iPhone 3GS. As iPhone 3GS doesn’t have front facing camera, so you will have to use the rear camera for this purpose.
FaceIt-3G is almost ready and the final bugs are being removed before it is released for public. You can watch the FaceTime working on a jailbreaked iPhone 3GS in this video: