Apple released the iPhone 3.1.3 firmware earlier today for iPhone and iPod touch and this updates baseband to 05.12.01 on iPhone 3GS and iPhone 3G, hence making it impossible for these two devices to get unlocked using blackra1n with blacksn0w. Fortunately for iPhone 2G (the first generation iPhone with EDGE) users, the baseband remains intact. The iPhone DevTeam has just confirmed that their newly released jailbreaking tool called redsn0w 0.9.3 can be used to jailbreak and unlock iPhone 2G on firmware 3.1.3, provided that you have jailbroken it previously at least once.
Update: redsn0w 0.9.4 released! Jailbreak iPhone OS 3.1.3 with redsn0w 0.9.4
WARNING: This works for iPhone 2G Only. Jailbroken and unlocked iPhone 3GS, 3G and iPod touch users should NOT upgrade until further instructions.
Redsn0w 0.9.2 final was released today along with Redsn0w 0.9.3 beta with internet tethering support is available for download for testing purpose only. You may proceed at your own risk. Just point Redsn0w 0.9.3 to 3.1.2 ipsw for iPhone 2G only.
You can follow the instructions posted here to jailbreak iPhone 2G with OS 3.1.3 using redsn0w 0.9.3
Download redsn0w 0.9.3 for Windows [Mirror link]
Download redsn0w 0.9.3 for Mac [Mirror Link]
Download the Original iPhone OS 3.1.3 firmware for iPhone