Although the UltraSn0w 1.2.5 is the old version used for 5.0.1 unlock but there is a UltraSn0w fixer tool or patch that enables UltraSn0w 1.2.5 to unlock some basebands on iOS 5.1.
Ultrasn0w Fixer has been released by a person, zmaster who released modified Ultrasn0w Fixer that enables the old Ultrasn0w 1.2.5 to unlock iOS 5.1 but there are sitll some limitation as far as baseband is concern so it supports the below basebands only:
Note: T-Mobile USA users have to disable 3G network before starting unlocking iphone Go to Settings >General >Network and switch the “Enable 3G” switch to OFF.
Remember your device should be Jailbroken so that you can use Cydia for unlocking using UltraSn0w 1.2.5, you can visit this article to jailbreak now.
Steps for Unlocking are:
Step 1: Tap Cydia icon to start the application and tap Manage after Cydia loads.
Step 2: Tap Sources
Step 3: Tap the Edit tab as shown in below screen
Step 4: Tap the Add Button
Step 5: Type following URL and Tap Add Source
Download of Cydia source will be done automatically. After the source is successfully added to the Cydia tap return to Cydia button at bottom of the screen and then tap the Done button. You will find the Ultrasnow Fixer utility in the list of sources in step 3 install it and wait till it complete the process. Go back to Cydia.
Step 6: Now you will be in the main window of Cydia if not just open it and tap Search at lower right corner. Now type Ultrans0w in Cydia search box as shown below and tap the ultrasn0w 1.2.5 as shown in below screen.
Step 7: After tapping you will see the below screen now tap Install and tap Confirm on next screen. The installation will begin and complete itself.
Step 8: Reboot your device.
Congratulations ! your device is unlocked on iOS 5.1 try using the different SIM cards.