Apple artificially limits the use of Facetime—their videoconference software for the iPhone 4—to Wi-Fi use. Good thing we have independent developers to make it work under 3G too. This is how to do it:
1. First you need to jailbreak your iPhone 4. This is a very easy procedure: Just go to this page and follow the instructions on screen. You don't have to install any software and there is no danger in doing this (you can always restore your iPhone 4 to factory settings with iTunes). [The webpage is down right now]
2. Go to this page and download the My3G program. This is what makes possible to FaceTime with the 3G network. It only costs $4 and you can try it for free.