iPhone firmware 3.1.3 resulted in upgrading iPhone 3GS and 3G baseband to 05.12.01 which is not unlockable at the moment hence a lot of iPhone users loose their iPhone unlock by upgrading their firmware to OS 3.1.3.
Update: UltraSn0w 0.93 is Out Now! It can unlock all basebands for iPhone 3GS and 3G. (Check the Updates at the bottom)
Discovery of this new exploit to unlock iPhone Baseband 05.12.01 is, indeed, a good news for the iPhone unlockers. For those thinking of this exploit as a fake, the most respected Dev-Team member, MuscleNerd, has confirmed that it works. The exploit has not been revealed yet and sherif_hashim has tweeted that 5.12 unlock release is totally the dev team opinion.
The iPhone Dev Team will most likely wait for the release iPhone 4G, the next generation iPhone, before releasing iPhone Baseband 05.12.01 unlock because apple will surely patch it if the exploit is released earlier. All the iPhone jailbreakers and unlockers should be cautious of any firmware update from Apple until there are jailbreak and unlock tools available for the latest update.
Stay Tuned for more on this issue…
GeoHot has an Exploit to unlock iPhone Baseband 05.12.01
MuscleNerd confirms Baseband 05.12.01 unlock:
Unlock iPhone Baseband 05.12.01 [Coming Soon]