The US mobile market is weird in that almost all the phones floating around here are locked to one carrier or another. You can usually request an unlock code from the carrier if you are not under contract or still paying off the device, but Sprint is different. It claims it doesn't have any mechanism to unlock phones for use on other US carriers right now, but that's going to change next year. Even in cases where you can get a SIM unlock on your Sprint phone, it will only work internationally right now – it's the only carrier that operates in this way. The CTIA's Consumer Code for Wireless Service was put together late last year and includes, among other things, requirements that carriers adopt more consumer-friendly unlocking procedures. The FCC had to do a bit of threatening to get this passed, but Sprint and other carriers got on board. Note: unlockable doesn't mean they are always unlocked, just that it is possible to unlock them, provided you qualify.