Geohot has put out a new jailbreaking solution for OS 3.1 & 3.1.2 that jailbreaks all iPods and iPhones including the ipt3. Windows & Mac support available.
for the download go to:
http://blackra1n.com
To use it make sure you have 3.1.2 installed.
Connect your iPhone to the computer open up blackra1n and click “make it ra1n”. Wait 15 seconds and your done.
If you run blackra1n and it takes forever reboot your PC and it should work.
If you are on ipt3 or have the newest bootrom iPhone 3GS you will have to reconnect to the computer everytime you reboot so make sure you are watching your battery so that you dont end up with a brick until you can find a computer with blackra1n.
To test if you are on the newest bootrom just turn off your phone after you jailbreak, if it comes up with the recovery screen you have the newest bootrom. Currently GeoHot is working on a solution to this, I will update if and when there is a solution.