As we all know, after releasing the jailbreak software for iPhone 3GS, Geohot also released his unlock method for iPhone 3GS.
Purplesn0w developed by Geohot is an alternative to ultrasn0w developed by the iPhone Dev Team to unlock iPhone 3GS. Geohot claims that his unlocking software actually improves on some of the common issues faced by using the ultrasn0w developed by the iPhone Dev team.
3GS Users can't restore to iPhone OS 3.0 as Apple has stopped signing iPhone
OS 3.0 and iPhone 3.0.1 firmware files after releasing iPhone OS 3.1 so you can no
longer use this guide to jailbreak your iPhone 3GS.
Use this guide instead: