iH8sn0w just released a new version of Sn0wBreeze, this version is made to jailbreak firmware 3.1.3. Sn0wbreeze is a windows desktop application, that looks like the PwnageTool for the Mac.

Sn0wbreeze v1.3 enables you to update your firmware to version 3.1.3, without updating your current baseband which will allow you to unlock after.

Warning: All iPhone 3GS and iPod Touch 2G owners, go to settings –> General –> About on your device and check your model number. If it starts with “MC” do not try this jailbreak! That means that you have the new boot rom.

Firmware 3.1.3 (you wil need this to jailbreak):
iPhone 3.1.3 firmware for iPhone 2G
iPhone 3.1.3 firmware for iPhone 3G
iPhone 3.1.3 firmware for iPhone 3GS
3.1.3 firmware for iPod Touch 1G
3.1.3 firmware for iPod Touch 2G

Sn0w Breeze can be downloaded here: Sn0wbreeze 3.1.3 for Windows

Also make sure you have installed .net framework 3.5, this is required to run Sn0wbreeze.
Download .NET Framework 3.5