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.
The new version of blacksn0w by msft.guy is said to correct Wi-Fi bugs as well as unlock the older baseband under firmware 3.1.3. GeoHot, the original developer of blacksn0w, has not commented on this release. The package will be available from the MMI repository, however if you must unlock your iPhone immediately just add the repository below:http://cydia.pushfix.info/
In January, iPhone Dev-Team released PwnageTool 3.1.5 for Mac OS X. This tool preserves your 3.1.2 jailbreak and ultrasn0w unlock when upgrading to firmware 3.1.3. We're guessing if you are interested in running blacksn0w RC2 you were already jailbroken but never bothered to download ultrasn0w.
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Unfortunately, as of yet there's no way to jailbreak your iPhone running 3.1.3 if you used the official Apple firmware or if you have a device with factory-installed 3.1.3 firmware.