Developer George Hotz has beat the iPhone Dev-Team in delivering the first Jailbreak for the iPhone 3GS. The Purplera1n jailbreak software as it is called seems to work as well.

This one runs on Windows XP and Vista (Windows 7 and Mac OS aren't supported, apparently - it's weird for Windows 7). The jailbreak assumes that your iPhone runs with OS 3.0 (for a 3GS, that should be the case) and that you have the latest version of iTunes (may be do an update check?). The author warns that the Jailbreak is a beta, so do a full backup before running it just to be safe. Obviously, do this at your own peril, as this is clearly not supported by Apple. Good luck.

[Ubergizmo]