The Dev Team has updated their UltraSn0w iPhone unlock for iOS 4.2.1. UltraSn0w 1.2 will unlock iPhone 4 baseband 01.59.00 and iPhone 3G/3GS basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04 and now 06.15.00.

If your iPhone 4 is not on baseband 01.59.00, you will not be able to unlock it. If your iPhone 3G or 3GS is on one of the baseband version mentioned above, then you can follow the instructions below…

If your iPhone 3G or 3GS is not on one of the baseband version mentioned above, you will have to first update your baseband to 06.15.00. See our PwnageTool tutorial or RedSn0w guide to do this.

This process doesn’t come without its share of risks, so it is very important you read this before doing anything. Seriously, make sure you read it.

Step 1: Make sure your iPhone is jailbroken using PwnageTool or RedSn0w.

Step 2: Launch Cydia, and search for UltraSn0w.

Step 3: Download UltraSn0w.

Step 4: Reboot your iPhone.

Your iPhone is now unlocked. There is no real way to test it until you pop a different SIM card in.