The ultrasn0w unlock for iOS 4.2.1 has been released and it works to unlock iPhone 3GS and iPhone 3G running iOS 4.2.1 on basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04 in addition to 05.14 and 05.15 via updating to 06.15.00 iPad baseband. Using the ultrasn0w unlock isn’t complicated, but it’s a multi-step procedure that carries a few risks and first requires you to jailbreak your iPhone. Before you get started with the unlock, you need to consider the following:

Important notes about iOS 4.2.1 unlock on iPhone 3GS and iPhone 3G

Unlocking your iPhone voids your warranty from Apple

For basebands 05.14 and 05.15 this iOS 4.2.1 unlock requires you to update to baseband 06.15 from iPad 3.2.2 firmware onto your iPhone, this can not be reversed

You can not downgrade from baseband 06.15 (the iPad baseband), and you will no longer be able to restore to stock firmware. This means you will forever need to use your custom saved IPSW files!

And finally, directly from the iPhone Dev Team: “iPhone3GS users with old bootroms who want to go to iOS 4.2.1 should not use PwnageTool! Update to stock iOS 4.2.1 first (through iTunes) then use redsn0w 0.9.6b5 to update your baseband.”

It is important that you understand and accept these risks before proceeding.

How to unlock iOS 4.2.1 on iPhone 3GS and iPhone 3G using ultrasn0w

If you have older baseband, you can go straight to using ultrasn0w. You can update to iOS 4.2.1 from iTunes but this will update your baseband and require the use of the iPad firmware. Here are the steps to unlock, the process is basically the same with Pwnage or redsn0w:

If you are using PwnageTool, Basebands 05.14 and 05.15 need to download this iPad IPSW file in addition to PwnageTool 4.1.3 Unlock Edition (Mac)

You can also just download redsn0w 0.9.6b5 for Mac and Windows

If using PwnageTool to create a new custom IPSW, use the aforementioned IPSW download (yes its for iPad).

If using redsn0w 0.9.6b5, the app will automatically download the iPad IPSW for you

Jailbreak your iPhone 3G or iPhone 3GS, pointing it at the newly created custom IPSW

After your iPhone has been jailbroken, reboot the device and then launch Cydia

Tap on “Manage” and then tap “Sources”

Tap to “Edit” and then “Add” a repository, and then type the following: http://repo666.ultrasn0w.com

After the repository has been added, you can search for “ultrasn0w” and download version 1.2

Download and install ultrasn0w 1.2, this will automatically unlock your iPhone 3GS and iPhone 3G

Restart your iPhone and enjoy your unlock

The process of jailbreaking and unlocking iPhones usually sounds more confusing than it is, just follow instructions carefully. The main problem with this unlock method is that certain baseband versions require the use of an iPad baseband which can not be reversed, this marks your iPhone in an obvious manner to Apple and this is why it voids your warranty with them. Jailbreaking is not illegal but it is frowned upon by Apple, however the difference between a standard jailbreak and this particular unlock is that jailbreaking is reversible, and this unlock is permanent.

Update: the iPhone Dev Team forgot to include the proper bundle for iPhone 3GS users on 4.1 with PwnageTool 4.1.3, but you can fix it rather easily. Here are their comments on the matter:

There’s an error in the bundle for the iPhone3GS 4.1 that prevents the new baseband from being used. If you know your way around OSX, please download the fixed bundle, and unzip it if Safari hasn’t already done so. Then “Show Package Contents” of PwnageTool.app, navigate to Contents->Resources->FirmwareBundles and drop it there. Otherwise, please wait for the updated PwnageTool, or the OSX version of redsn0w coming soon.

Update 2: The redsn0w 0.9.6b5 download is now available for Windows and Mac, it’s an easier method to perform the jailbreak and unlock for many.