Folks who have upgraded their unlocked iPhone 4 or iPhone 3GS to the latest iOS 5.1, must be aware that unlocking tool UltraSn0w is not compatible with iOS 5.1. But no need to worry there is another unlock tool available for jailbroken iOS 5.1 iDevices named UltraSn0w Fixer.
UltraSn0w Fixer is a third party temporary unlock solution available through Cydia but at the moment it works to unlock jailbroken iOS 5.1 iPhone 4 and iPhone 3GS with following basebands.
To hand over UltraSn0w Fixer just find following repo over Cydia.
How to Unlock iPhone 4, 3GS over iOS 5.1
Jailbreak iPhone 4, iPhone 3GS over iOS 5.1 with RedSn0w
Jailbreak iPhone 4, iPhone 3GS over iOS 5.1 with Sn0wBreeze