Jon Lech Johansen, better known as DVD Jon, has started working on a way to unlock the iPhone so it can be used without an AT&T contract.
So far he has found a way to enable to music and WiFi parts without an AT&T contract but calling still doesn't work.
You can find more info at his site:
Download Phone Activation Server v1.0 to activate your iPhone for iPod+WiFi use. Note that this application will not do anything unless you understand the magic numbers as well as add the hosts entry. Phone Activation Server (PAS) requires that you have the MS .NET Framework 2.0 installed.