Apple artificially limits the use of Facetime - their video conferencing 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. There is no danger in doing this (you can always restore your iPhone 4 to factory settings with iTunes).
2. Go to this page and download the My3G program. This is what makes possible to FaceTime with the 3G network. It only costs $US4 and you can try it for free.