A new tool by xda-developers member helroz does what various companies on the internet will charge you $10-40 for. SIM unlocking allows you to use a different carrier’s SIM card in the phone, and also can increase the resale value of your phone. It works on 2.1 or 2.2 devices, and essentially fool the phone into thinking it’s already unlocked, rather than actually needing the unlock code. Of course, Vibrant users may know that if you’ve been a T-Mobile customer for a while, they will give you the unlock code for free. You mileage may vary with other carriers.
If you’re rooted and have busybox installed, head over to the xda-developers thread for downloads or to provide feedback.