1) Jailbreak your iPhone 4 via www.jailbreakme.com.
2) Download and upgrade to the latest Cydia package.
3) Open Cydia, Go to Manage then Sources.
4) Click Sources, Edit then add: http://apt.modmyi.com
5) Search for “My3G” — download, and install it.
6) Go through the Rock setup process with a username and password. Open My3G and leave all settings on the default.
7) FaceTime should now work over 3G! — Make sure to “Forget” the WiFi network you are on (via iOS settings) if you want to test it. “
When options and functionality that is supposed to be locked down suddenly becomes very easy to do on a Jailbroken phone, I often wonder if this is the way Apple wanted it to happen in the first place. I sometimes think that AT&T comes to Apple just before launch with a list of stuff they suddenly don’t like and demands it gets stripped out. The engineers probably just turn it off allowing for a little guessing work from the outside developers and hackers.
Without wanting to take anything away form the iPhone Dev team, the browser based Jailbreakme.com seems like a nice way for Apple to allow certain users to switch networks and regain the functionality they intended to deliver in the first place.