The iPhone dev team is yet to come out with PwnageTool and redsn0w to jailbreak iPhone OS 3.1.2. However, if you have eagerly awaiting to jailbreak your iPhone, the GeoHot team has released BlackRa1n, which will allow Windows users to jailbreak their iPhone with OS 3.1.2.
The current update of Blackra1n only supports iPhone OS 3.1.2, do not use this tool if you are using an earlier version of iPhone OS.
You can download the jailbreak software for iPhone OS 3.1.2 from here. Once you have downloaded Blackra1n, make sure to update your iTunes to the latest version.
Once you attach your iPhone, you will be prompted to upgrade to the latest iPhone OS 3.1.2. Update your iPhone to the latest version and click on the Blackra1n executable, click on the make it ra1n button to jailbreak your phone.
Your phone will automatically be put in recovery mode while it is jailbroken. It will automatically reboot after a while. To complete the process download the Cydia application from Blackra1n and install it.
Update: Blackra1n is also able to jailbreak iPhone OS 3.1. Thanks to commentator SimBech for confirming this.