T-Mobile branded phones operating on the T-Mobile network may only use T-Mobile SIM cards. To use a T-Mobile branded phone on another wireless carrier's network with a non-T-Mobile SIM card, you must first request a "SIM Unlock" code from T-Mobile. Conversely, if you have a phone from another North American wireless carrier, you may need to request a "SIM Unlock" code from that carrier before being able to use a T-Mobile SIM card on the T-Mobile network. * SIM Unlock Codes will only be provided after your account has been active for 90 days. Only one unlock code may be provided per customer every 90 days.Yes, even if you pay $399 for a contract-less phone, you've still gotta have an active account for 90 days to have T-Mobile unlock it. Of course, as we noted before, if you're just interested in unlocking the G1 you really won't have to worry about this at all: Developers can (and probably will, we're betting within a couple of days after the launch) totally put an unlocking application in the Android Market—it won't be blocked or pulled down. Definitely a prudent and laudable move on T-Mobile's part, since trying to fight hackers on a phone almost explicitly designed for it would be retardo to the tenth degree. See, it really is up to devs to make Android awesome. [T-Mobile G1 Coverage @ Giz]
One of the G1's five most obnoxious flaws (okay, there were more than five) is that it's locked to T-Mobile, which seems antithetical to the whole openness deal. Since T-Mobile is subsidizing the phone so heavily, it originally sounded like T-Mobile's usual unlock policy wouldn't apply, and a spokesperson wasn't sure when we asked yesterday. Good news though: The official word from T-Mobile is if your account has been active for 90 days, they'll unlock the G1 for you, just like they would with any other phone.