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The iPhone Dev Team (http://blog.iphone-dev.org/) has released an updated version of ultrasn0w which allows AT&T users who have already unlocked their iPhones for use with different wireless carriers to upgrade to iOS 4.3.1 without losing the carrier unlock.
The latest tool isn’t designed to work with newer phones with the latest basebands. Instead it’s designed to let users who have already unlocked their phones upgrade to the latest iPhone operating system without losing the ability to use their phone on their network of choice.
In order to use ultrasn0w, first you have to jailbreak your iPhone 3, 3GS, or iPhone 4 and install the Cydia app store. Then you download and install ultrasn0w 1.2.1 and restart your phone. It’s pretty much that easy — but you can find step by step instructions and pictures (http://www.redmondpie.com/unlock-ios-4.3.1-iphone-4-3gs-using-ultrasn0w-how-to-tutorial/) at Redmond Pie.