Today, in yet another surprise break through, jail breakers succeeded in facilitating iPhone 3G and 3GS users to unlock their iPhones on the newly released version of OS 3.1.3 using Sn0wbreeze. Redmondpie has in its latest post has chalked out a comprehensive procedure whereby following that user can simply unlock their iPhone and upgrade it with the latest version of OS 3.1.3. 

sn0wbreeze - V1.3

According to wesite, sn0wbreeze has now been released which can jailbreak iPhone 2G, 3G and iPhone 3GS on the latest iPhone 3.1.3 firmware. One thing which is to be noted here is that it will work with iPhone 3GS only if it has the older bootrom (non-MC model). iPhone 3GS users who have upgraded to the stock 3.1.3 are still out of luck. For detail step by step upgrading procedure, follow instructions laid down here.