UPDATE:
Gevey has released the Ultra 5.1 to unlock the GSM
The new GEVEY Ultra unlocks your GSM iPhone 4 running iOS 5.1 and earlier and now supports modem firmware versions 1.59, 2.10, 3.10.1, 4.10.1, 4.11.08, and 4.12.01. Currently, a tethered Jailbreak is available for iOS 5.1 for iPhone 4, and you will also need to install the latest Furious Mod file for proper operation.
You can purchase the GEVEY Ultra 5.1 for $49.99