Blacksn0w RC2 is now available for those looking to unlock their iPhones running firmware 3.1.3. Beware however, this will not work with the latest baseband version 05.12.07. If you've updated your iPhone to 3.1.3 using Apple's official firmware you are out of luck.
Those updating their jailbroken device to iPhone OS 3.1.3 using PwnageTool 3.1.5 have taken advantage of Apple's battery fixes without altering the baseband from its current unlockable version, 05.11.07. In this case you can now download blacksn0w RC2 from Cydia.