We’ve pushed out the 4.3.1 compatibility fix for ultrasn0w in Cydia — it’s now at version 1.2.1. If you’re not already at 4.3.1 and you need the unlock,please be sure you understand how to get to 4.3.1 using a custom IPSW that doesn’t update your baseband.
This isn’t a new unlock! It’s to allow those who are already unlocked to use FW 4.3.1. It also fixes the signal bar issue for those who aren’t using the unlock but retain an older baseband intentionally.
AFTER INSTALLING ULTRASN0W 1.2.1, PLEASE REBOOT YOUR iPHONE using the normal “slide to power off” swipe. T-Mobile users in the USA also should disable 3G mode in Settings->General->Network.
If you are iPhone 3GS user then first you have to update your iPhone 3GS baseband to 06.15.00. Follow the steps posted here to update your iPhone 3GS baseband to 06.15.00 with PwnageTool or you can follow the steps posted here to update your iPhone 3GS baseband to 06.15.00 with Redsn0w. Now you have to jailbreak your iPhone 4, 3GS on iOS 4.3.1 and preserve the old baseband using Sn0wbreeze on Windows or PwnageTool on Mac. Follow the steps posted here to jailbreak iPhone on iOS 4.3.1 using Sn0wbreeze. Follow the steps posted here to jailbreak iPhone on iOS 4.3.1 using PwnageTool. After jailbreaking your iPhone 4, 3GS simply follow the steps posted below to unlock iOS 4.3.1 on iPhone 4, 3GS using Ultrasn0w 1.2.1.
How to Unlock iOS 4.3.1 on iPhone 4, 3GS Using Ultrasn0w 1.2.1
STEP 1. Launch “Cydia” on your iPhone, then tap on “Manage” tab.
STEP 2. Now tap on “Sources”.
STEP 3. Now tap on “Edit” and then tap on “Add”. Now you will be prompted to add a URL source. Now type “http://repo666.ultrasn0w.com” and then tap on “Add Source” to add this repository.
Now Cydia will automatically update your sources.
STEP 4. Once the installation is completed, simply search Ultrasn0w 1.2.1 in Cydia and then install it. Now your iPhone 4, 3GS will be unlocked automatically.
Now restart your iPhone. That’s it. You have successfully unlocked your iPhone 4, 3GS on iOS 4.3.1. In this way you can unlock iOS 4.3.1 on iPhone 4, 3GS using Ultrasn0w 1.2.1. [Via]