The latest and greatest jailbreak tool has been released and it supports iPhone OS 3.1 on the 3GS hardware. Before you get too excited, keep in mind it will only work if you've already jailbroken your iPhone 3GS when it was running 3.0 or 3.0.1 firmware.
Turns out if you've installed an update directly from Apple using iTunes on your iPhone 3GS or the device came pre-installed with firmware 3.1 then you're out of luck. PwnageTool 3.1.3 is only available on the Mac platform. Dev-Team has stated it has no intention of developing a windows version of this particular tool.