AT&T released their information yesterday about selling unlocked iPhones in the States.
Unlock Your Own Iphone: Don't Buy an Unlocked iPhone From AT&T, Just Cancel a New Contract
It seems it would be cheaper to sign the contract and buy out of it 30 days later than it would be to just purchase the thing outright from them.
If you don't want to click the link, the breakdown is:
8GB iPhone Canceled: $480
16GB iPhone Canceled: $580
8GB iPhone Unlocked from AT&T: $599
16GB iPhone Unlocked from AT&T: $699