The iPhone Dev Team has just released PwnageTool 3.1.4 for Mac OS X. This release supports the 3.1.2 iPhone software for the following devices: iPhone 2G/3G/3GS and iPod Touch 1G/2G.The iPhone 3GS is now supported out of the box in PwnageTool 3.1.4 (or if you have upgraded to 3.1.x in iTunes)The iPod 2G is still supported in PwnageTool 3.1.4 but you must already be jailbroken. The iPod touch 3G is NOT supported.
According to the Dev-Team, iPhone 3GS users can jailbreak and unlock iPhone OS 3.1.2 using PwanageTool, but there is a catch: it will only keep the unlock for the cellular baseband intact if you have been jailbreaking and unlocking your iPhone 3GS using custom IPSW files beginning with iPhone OS 3.0 or earlier. (If that describes your situation, all you have to do is install ultrasn0w via Cydia after you perform a restore using one of the no-baseband-alterations custom IPSW's you created previously.)
The Dev-Team advises that 3GS users get and save their ECID hashes for iPhone OS 3.1.2 now, since this could be an important component of future jailbreaking efforts. If you have installed Blackra1n using our tutorial and are encountering problems with mobilesubstrate, winterboard, diskaid, or ifunbox, the Dev-Team has some custom IPSWs that can help. They also warn Blackra1n iPhone 3G and 3GS users that if the stock 3.1.2 firmware is used, there is no short-term unlocking solution. The Dev-Team is still working on hacks for the latest iPhone basebands.
GOLDEN RULE: If you are using a iPhone 3G or iPhone 3G(S) with ultrasn0w and rely on ultrasn0w to obtain cellular service then you should only update your device with an .ipsw that is made with the new PwnageTool. There are no second chances with this. You need to remember that PwnageTool will provide an upgrade path to newer versions of the iPhone software in the future.