Owners of unlocked iPhones who want to upgrade to the recently released OS 3.1.3 and want to retain the ability to use whatever Sim meets their needs can now do so. The iPhone Dev Team have released a suitable version of its PwnageTool utility.

The Team cautioned against updating. The iPhone OS update is claimed to improve the accuracy of the iPhone 3GS’ battery level readout, and if you’ve not noticed any untoward readings, the Team say, “there’s no reason to update to 3.1.3″.

If you must, PwnageTool 3.1.5 is what you need.