The Dev Team have uploaded PwnageTool_3.1.4.dmg to their torrent site. This tool will allow you to create a custom jailbroken 3.1.2 firmware on your Mac for shift+restoring to your device. As with the previous versions, PwnageTool 3.1.4 will work on all devices except for the iPod touch 3G and there is no Windows version. The Pwnage tool can also hack in an older baseband to allow unlocking and hacktivation of iPhones.
It can never be said too many times: if you need an unlock DO NOT upgrade to a stock Apple firmware >3.0!