The guys and gals over at xda-developers.com have found an easy way for you Galaxy S Vibrant or Captivate owners to unlock your phone. Instead of storing the encrypted unlock codes on some remote server that would make it difficult to find, the codes are located directly in your Galaxy S in a BAK file on the phone's internal SD card.
So in essence, if you have the Galaxy S Vibrant from T-Mobile you can use it on AT&T's network, and vice versa. Reports from many users are saying the Captivate will only operate on T-Mobile's EDGE network, though.
To unlock your device, you'll need some knowledge of ADB, and be able to open the bml3.bak file in a hex editor. The forums come in very handy for this here. If that sounds like too much work for you, there is an app in the Market that can be downloaded for $9.99 called Galaxy S Sim Unlocker which will decode the unlock key for you.
There is no reason to be worried about breaking your warranty with Samsung, the phone can easily be locked to it's original setting with just a few steps. Big thanks to xda-developer forum members rhcp0112345 and galaxysguy for making this discovery, and sharing it with the rest of us. The Galaxy S Sim Unlocker can be downloaded by scanning the QR code below with your phone.