The DevTeam has just released Pwnage Tool 3.1-3, the new version of the tool for Mac OS that lets you run the jailbreak on iPhones and iPod Touch with firmware 3.1. The new version adds limited support for iPhone 3GS and iPod Touch 2G. It still works with iPhone 2G (EDGE), iPhone 3G and iPod Touch 1G.
The iPhone 3GS is now supported in PwnageTool 3.1.3, assuming the phone was pwned at 3.0 or 3.0.1 – PwnageTool does not support the 3GS out of the box. If your iPhone 3GS has 3.1 preinstalled and is not Pwned then there is no tested jailbreak solution at the moment.
You can download in Pwnage Tool 3.1-3 here.