It is now possible to carrier unlock all GSM varieties of the Samsung Galaxy S, according to a detailed thread at the XDA Developer forums. Usually third party services charge up to $30 for the privilege of opening a handset up for use on unintended carriers, so the free method cooked up by “dagentooboy” and others should prove popular assuming that it is stable.
Unfortunately for the average user, the unlock procedure is a bit complicated, and should not be attempted by those uncomfortable with HEX editors or entering command line script. It should also be noted that just because a phone is physically capable of running on multiple networks, it may not support the proper bands for full 3G coverage. Practically, this means that unlocking a Vibrant from T-Mobile and setting it up on AT&T will result in 3G coverage only where the latter carrier offers 1900MHz spectrum.
Finally, there have been some reports of devices freezing after too many unsuccessful unlock attempts, so this hack should be attempted at your own risk.