Apparently, the amazing code wizards at the iPhone Dev Wiki have been able to partially unlock the iPhone using a new application called iASign. It won't fully unlock the iPhone for use with other companies, but the hack will allow you to use any existing Cingular/AT&T Pre-paid/MVNA SIM so you don't have to get a two-year contract with AT&T. We are now testing this, but if confirmed the benefits are great.
http://gizmodo.com/gadgets/breaking/iphone-partially-unlocked-calls-without-att-contract-279606.php