The iPhone dev team has released a video demonstration of the iPhone 3G unlock, which is due to be released on December 31, 2008.
The video is embedded below, but can also be seen on the iPhone dev team blog. It shows an AT&T SIM-locked iPhone 3G being unlocked, then successfully accepting a T-Mobile SIM card and receiving a call.
The unlock process looks pretty simple, requiring only a script to be run from a connected computer (a system running Mac OS X in the video). "MuscleNerd," who posted the video, claims that a daemon will obviate the need to enter any terminal commands in the final release of the unlock tool.
This unlock method will be available only to iPhone 3Gs that have 2.11.07 baseband or earlier--this is the baseband version that shipped with iPhone OS 2.1. You can tell what version baseband you have by going to Settings->General->About->Modem Firmware.