It's my feeling that the hackint0sh community stands a good chance of being one of the first to unlock the iPhone.
Here's where they stand today:
The iPhone SIM, once activated seems to work fine in unlocked and newer and older AT&T/Cingular phones that are locked. Other AT&T/Cingular SIM cards will not work until activated with iPhone plans. They connect, can't dial. iTunes will let you activated it. Non AT&T/Cingular SIM cards will not work at all.
So, this phone has almost a double-lock on it. You have your standard GSM subsidy lock, but you also have some sort of mechanism that is tying the SIM card to the phone.