The Blackra1n jailbreak has been released for iPhone OS 3.1.2 devices including the iPod touch 3G and new iPhone 3GS.
blackra1n is a 30 second ALL device 3.1.2 jailbreak. Even the ipt3, but the ipt3 is tethered. In order to boot it, just rerun blackra1n.
-- but you can't unlock yet, so don't get any big ideas about switching carriers. Windows-only at the moment, but with this level of enthusiasm out there we can't imagine a Mac version is far behind.
Geo also warns 3G and 3GS unlockers not to upgrade to 3.1.2 using iTunes if you want to keep your unlock as the baseband cannot be downgraded.
Unfortunately Blackra1n is only available for Windows, for now.
Update: Blackra1n jailbreak is now available for Mac OS X.