As you know the Dev team has jailbroken the iPhone 3GS using a modified version of Redsn0w(please note at the time of writing this article, redsn0w is not compatible with the iPhone 3GS). Since the iPhone 3GS has the same baseband as the iPhone 3G, Ultrasn0w will work to unlock it. Here is a demo of the unlock which should be available soon after the release of the iPhone 3.1 firmware.

iPhone 3GS Unlock Demonstration from planetbeing on Vimeo.