If you are one of those who has been waiting for a jail breaker to be available for iPhone 3.1.2 before upgrading, now it’s time.
This time it appears that the developer has made the jailbreak available only on windows. You can download it here. Be aware though that jail breaking is hit and miss, so make sure you backup as if this go wrong, you may end up with brick!
This is what blackra1n.com has to say:
So, what is blackra1n? 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. As far as donations go, they are much appreciated, but not until after blackra1n works for you. I never ask for donations before the tool is released.
Warning 3g and 3gs unlockers, do not upgrade to 3.1.2 using iTunes if you want to keep your unlock. Baseband cannot be downgraded. Check out the dev team’s offerings. Also blackra1n doesn’t hacktivate.