Samsung Vibrant's is a T-Mobile device and Samsung Captivate belongs to AT& T, but you can swap them around -- all you need is a simple script and a USB cable to completely unlock your phone. A cracker-jack team at XDA-Developers discovered that Galaxy S unlock codes aren't kept in a secure remote server, but merely stored in a .BAK file on the device itself, which you find with a hex editor and key right in when inserting a new SIM card of your choice.
"We tested it out on both Vibrant and Captivate and were up and running in less than five minutes each time, and bonus, Vibrant gave us a 3G data connection (with 2Mbps down) using an AT& T SIM,".
The best part? None of this requires you to actually root the phone, and if you're worried about warranty you can re-lock the handset with the very same code.