Users have been jailbreaking devices with iOS 4.2.1 since day one. But while jailbreaking lets you run apps that aren’t available in the App Store on an iPhone or other Apple mobile device, there’s one thing it doesn’t automatically do: Let you use an iPhone in the US with a carrier other than AT&T. That’s something you need to unlock your phone to do, and up until now there hasn’t been an unlocking tool for iOS 4.2.1
It looks like that could change this weekend. Hacker MuscleNerd reports that he managed to get the ultrasn0w unlock tool working on iOS 4.21, and he’s posted a picture showing the About screen of an iPhone running iOS 4.2.1 and operating on the T-Mobile network.
Right now, it only works on the iPhone 3G and 3GS. It might be a while before we see support for the iPhone 4.
But if you’ve been holding off on updating your iPhone to iOS 4.2.1 because you don’t want to lose your carrier unlock, it looks like you might not have to wait much longer.