Just a couple of hours back, the iPhone Dev Team has updated their carrier unlock tool which brings unlock for iPhone 3GS & 3G on iOS 4.1 or iOS 4.2.1 but unfortunately, you have to update your device’s baseband to 06.15.00 to be able to unlock it.
Simply follow the steps below to jailbreak and unlock your iPhone 3G on iOS 4.2.1 using PwnageTool 4.1.3 for Mac OS X.
STEP 1: Download PwnageTool 4.1.3 unlock edition from here.
STEP 3: Now launch PwnageTool and select iPhone 3GS / 3G and point it to the original iOS 4.2.1 firmware you downloaded.
STEP 4: You’ll now be asked if you want to update your baseband to the 06.15 version. If you are on 05.14 or 05.15 or higher you will need to do this to unlock. You do not need to do this for jailbreak only. Click on YES and then select Browse for IPSW and point it to iOS 3.2.2 you downloaded.
STEP 5: You’ll have to choose some options as installing Cydia packages, custom logos and packages. Finally, hit the blue the build button.
STEP 6: You’ll be asked to save the cooked firmware. Please allow up to 10 minutes.
STEP 7: When prompted if your iPhone has been Pwned before, we clicked No. It allows for a more thorough restore. Once done, follow the instructions seen on the screen to enter into DFU mode
STEP 8: Now, You will have to restore your iPhone to the jailbroken custom firmware by Clicking on the iPhone icon on the left sidebar of iTunes then hold left “alt” button (“Shift” button on Windows) on the keyboard and then click on “Restore” then release the alt button, Now Select the custom firmware created by PwnageTool and Let iTunes go in a bunch of steps.
STEP 9: Almost done, you can now unlock your iPhone 3GS / 3G by following my previous guide posted here.