Just as we had expected, the iPhone dev team has released the Ultrasn0w 1.2 tool to unlock the latest iOS 4.2.1 firmware on iPhone 3GS and 3G on basebands later than 05.13.04. The trick is to flash iPhone 3G/3GS with 06.15 baseband from iPad 3.2.2 update.
Note that iPhone 4 is not unlockable yet since it has a different baseband OS. It is important to repeat the risks involved with unlocking iOS 4.2.1 using Ultrasn0w 1.2
– Once you unlock your iPhone 3G/3GS, you can never go back from 06.15 baseband. Warranty would be void and the baseband version won’t be hidden from Apple.
– If some future baseband comes out with a critical fix, you won’t be able to update to it if it remains down in the 05.xx sequence (then again, you wouldn’t update to it if you wanted to keep your unlock anyway).
– Starting with FW 4.2.1 iPhone 3G with 06.15 baseband, you won’t be able to restore to stock firmware (it will fail). Restore using custom IPSWs to retain your unlocks.
Ensure your iPhone 3G is fully charged and connected to the PC or Mac via USB through out the process. Also, do NOT leave your SIM module inside the iPhone. Eject it and keep it aside.
Step 1: Once jailbreak is successfully completed, connect your iPhone 3G/3GS to a Wi-Fi network and run Cydia.
Step 2: Go to Manage menu and select Sources. Tap on Edit button on the top right corner and then tap on the Add button on top left corner.
Step 3: Type http://repo666.ultrasn0w.com in the small Enter Cydia/APT URL pop-up window.
Step 4: Once the ultrasn0w source is added, tap on ‘Return to Cydia’ and choose repo666.ultrasn0w.com from the sources list.
Step 5: Look for Ultrasn0w 1.2, select it and tap on Install button on top right of the screen
Step 6: Wait for ultrasn0w package to be installed and tap on Restart the Springboard.
Step 7: Insert the active SIM you wish to use on this iPhone 3G running iOS 4.2.1 with 06.15 baseband.