PC Magazine reports: "Now that the Copyright Office has given a green light to unlock your GSM or iDen phone, you can save money on a new phone – provided you can find it unlocked, or unlock it yourself. Depending on your phone, you might be able to do that by punching in a code, by buying a cable, or by sending it to a professional unlocking service. This applies to Cingular, T-Mobile and Nextel phones; Sprint, Verizon and Alltel subscribers are out of luck, for reasons I'll explain below."
This is a great article on how to unlock several different types of phones. Some require codes, others cable and software. Some can't be unlocked at all.