Finally after long wait iPhone dev team has released Ultrasn0w 1.2 to unlock iOS 4.2.1 Baseband 05.15.04. However these times unlock hasn’t come with easy process. The unlock for the baseband prior to 05.14 was exploited the AT+XAPP command, thanks to a crash initially discovered by @sherif_hashim (@Oranav also found this crash).
Unfortunately same hole has been closed in iOS 4.1 baseband 05.14.04 & iOS 4.2.1 baseband 05.15.04 making it impossible to do carrier unlock. Well not need to disappoint, thanks to iPhone dev team for their yet another unique way of getting Ultrasn0w unlock.
Since iPad iOS 3.2.2 baseband 06.15.00 still vulnerable to AT+XAPP. The iPad baseband is built for the exact same baseband chip as the iPhone3G/3GS. Hence the unlock is achieved by only updating baseband of iPhone 3G & iPhone 3GS iOS 4.2.1 to 06.15.00 and then using Ultrasn0w 1.2 to unlock the carrier.
You need to use updated version of Pwnagetool 4.2.1 for mac & Redsn0w (should be released any time soon) specially crafted to just update baseband of iPhone 3G & iPhone 3GS to 06.15.00 and then use Ultrasn0w 1.2.
As usual there are consequences to this method of Ultrasn0w 4.2 Unlock.
1. There is no way to come back down from 06.15, and there’s no hiding the baseband version from Apple. You’ll be voiding your warranty in a very obvious way.
2. 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).
3. Starting with FW 4.2.1 if you have 06.15 on your iPhone you won’t ever be able to restore to stock firmware (it will fail). You’ll need to only restore to custom IPSWs (then again, if you’re unlocker you should already be doing that).
Now if you fail to understand what all of above mean, you better wait for our iOS 4.2.1 unlock guides.