Unlock iPhone 4 on iOS 5.0 baseband 4.11.08 and iOS 5.1 baseband 4.12.01 is available by GEVEY Ultra Interposer.
GEVEY Ultra has support for iPhone 4 GSM running on iOS versions 4.1 to the latest iOS 5.1 and will work on following basebands-
Requirements to use GEVEY Ultra is that you need a jailbroken iPhone, an officially activated iPhone via iTunes before GEVEY Ultra is used and installation of the latest Furious Mod file.
GEVEY Ultra can now unlock iPhone 4 with modem firmware up to 4.12.01 with better battery life and enhanced signal strength for strong,consistent voice and data connection.
Currently, there is no untethered Jailbreak for iOS 5.1.However, you can jailbreak your iPhone 4 using latest version of Redsn0w unlock it using GEVEY.
Also Read: Unlock iPhone 4S With Gevey Ultra S
You can pre-order GEVEY Ultra 5.1 from the Apple N Berry website at a price of $49.99.Orders will be shipped by the end of April.