iPhone Dev Team’s Redsn0w 0.9.10b6 conveniently  jailbreaks iOS 5.1 firmware but does not support unlocking iPhone 4 and iPhone 3GS (on old basebands) using Ultrasn0w 1.2.5. Till the time hackers pushes out ultrasn0w unlock for iOS 5.1 compatibility, you can install Ultrasn0w Fixer Patch for iOS 5.1.

The package, “ultrasn0w fixer for 5.1,” patches the iPhone Dev-Team’s ultrasn0w unlock, to work with iOS 5.1 jailbroken iPhone. Developed by msftguy and zmaster, ultrasn0w fixer for 5.1 can be downloaded by adding the repo: http://repo.iparelhos.com. Supported modem basebands are listed below. Everything else is NOT supported.

• 01.59.00 (iPhone 4)
• 04.26.08 (iPhone 3GS)
• 05.11.07 (iPhone 3GS)
• 05.12.01 (iPhone 3GS)
• 05.13.04 (iPhone 3GS)
• 06.15.00 (iPhone 3GS)

Instructions to unlock iOS 5.1 using UltraSn0w Fixer for 5.1

Step 1: You must first Jailbreak iOS 5.1 firmware using Redsn0w 0.9.0b6. In order to proceed, be sure to preserve your baseband from being upgraded. To do so, create custom IPSW iOS 5.1. iPhone 4 and iPhone 3GS owners can use redsn0w’s “Custom IPSW” button to create a NO_BB_* version of the 5.1 IPSW and restore to that instead of the stock one. If you try restoring your iPhone to a stock Apple IPSW, your iPhone will be locked. If you are already running iPhone with preserved baseband, skip this step and hop to Step 2.

Steps to create custom 5.1 iOS firmware to preserve Baseband

• Launch Redsn0w, select “Extras”, and then select “Custom IPSW”.
• Locate your iOS 5.1 Stock firmware then the Redsn0w will run a number of background processes to create custom IPSW which is pre-jailbroken with preserved baseband.
• Navigate to Extras > Pwned DFU and restore to your custom iOS 5.1 firmware (created in step 2) using iTunes.

Step 2: After you have successfully jailbroken your device, launch Cydia from your iPhone Springboard.

Step 3: Go to the “Manage” tab, then tap “Sources”
Step 4: Tap Edit, then Add, you should get a dialog asking for a URL
Step 5: Enter this URL: http://repo.iparelhos.com and then press ‘Add Source’.

Step 6: After the repository has refreshed and finished loading sources, Tap on the iParelhos repo to see Ultrasn0w Fixer utility.

Step 7: Select the Fixer utility, and tap on the ‘Install’ in the top right hand corner.

Step 8: Once installation is complete, go ahead and install ultrasn0w 1.2.5.

Step 9: Reboot your iPhone and your iPhone should be unlocked on latest iOS 5.1 IPSW mobile operating system.