iPhone Dev Team have put up a video tutorial on unlocking the iPhone 3GS using ultrasn0w. The unlock will not work if you have already upgraded your iPhone to 3.1 by downloading the Apple IPSW file.

iPhone 3GS Unlock Demonstration from planetbeing on Vimeo.

Meanwhile, the iPhone Dev Team are still working on a new program to unlock the latest firmware release for the iPhone.

via iPhone Dev Team