Developers over at XDA Developers have managed to find that the range of Galaxy S class smartphones have unlock codes built within that allow you to switch networks in the US. To activate the Galaxy S unlock code you need a USB cable and a script to run that when successful, lets you run your Samsung Vibrant T-Mobile device on the AT&T network.
The Samsung Galaxy S unlock codes are stored in a .BAK file on the device. The normal place manufacturers keep unlock codes is on a secure server. Having them on the device it's self means that no real tampering with the system needs to be done to crack it open.
The Galaxy S unlock doesn't require root to be used, and can easily be switched back to locked if needed.
Things to remember when switching networks is that in the US, not all networks are compatible. T-Mobile and AT&T are, but if you are going from AT&T to T-Mobile then you wont get 3G speeds. Going in the reverse direction you will though.
Full details of the Galaxy S unlocking method can be found over at XDA Developers.