Over a year ago, the iPhone Dev-Team announced that they had found a way to unlock the iPhone 3G and 3GS by upgrading its baseband to the version found in the first-gen 3G iPad. It worked, but it had some kinks. The biggest drawback to using this method was the loss of the GPS functionality. This could be a big problem with iOS 6 on the 3GS, due to the heavy use of GPS capabilities. However, it looks like MuscleNerd was hard at work and found a way to downgrade the baseband from 06.15.00 to 05.13.04, which is fully unlock able and works correctly. This unlock works for both the iPhone 3G and 3GS with the baseband 06.15.00. There are also a few other disclaimers that MuscleNerd mentions in his blog post:

If you have an iPhone3GS and if digits 3-5 of its Serial Number are 134 or later (xx134…), then you should NOT try to install the 06.15 baseband on your 3GS!  It will brick your radio, preventing both the downgrade from working and normal iPhone software from using it as a phone!  Be warned!

You can check to see what baseband you have by going to Settings->General->About and scrolling down to “Modem Firmware”.

Redmond Pie has an excellent walk through on how to do it, that’s fully detailed and ready for pretty much anyone.

Via:  RedmondPie, iPhone Dev Team

This article is presented for informational purposes only. Jailbreaking can void Apple’s iPhone/iPod Touch/iPad warranty. Any efforts or attempts at jailbreaking is done at the sole discretion of the user. Today’s iPhone assumes no liability or responsibility for any resulting user actions, software or hardware glitches or revocation of warranties. Proceed at your own risk.