PwnageTool 3.1.3 is out! This latest version of PwnageTool updates the firmware to 3.1 on iPhone 3GS but preserves the older Modem version (Baseband) to 04.26.08 so that it can be unlocked using ultrasn0w and can be used with any carrier SIM.
NOTE: This guide is for those of you who are on a jailbroken firmware 3.0 or 3.0.1. If you have updated to firmware 3.1 from iTunes but have saved your SHSH file to Cydia then you can still jailbreak but you’ll first have to downgrade your firmware back to 3.0 or 3.0.1. Complete guide on how to downgrade your iPhone 3GS to firmware 3.0 or 3.0.1 from 3.1 can be found here. In any other case, you are pretty much out of luck for now.
Once you are up and running jailbroken firmware 3.1 on iPhone 3GS, follow the simple step-by-step instructions posted here from Step 4 onwards in order to successfully unlock iPhone 3GS on 3.1 firmware with ultrasn0w.
Complete tutorial/guide on how to jailbreak and unlock iPhone 3G and iPhone 2G running on firmware 3.1 can be found here.
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Feel free to ask me any questions here should you run into any problems when unlocking your iPhone 3GS.