Hot on the heels of comex’s JailbreakMe iPhone 4 jailbreak, the iPhone Dev Team have released an update to their famous Ultrasn0w Cydia software package allowing iPhone 4 owners to unlock their handsets and use them on a carrier of their choice.
Ultrasn0w 1.0-1 will unlock the iPhone 4 baseband 01.59 as well as 04.26.08, 05.11.07, 05.12.01 and 05.13.04 on the iPhone 3G/3GS.
Users will need to have jailbroken their phones before they attempt to unlock.
A word of warning. Before you unlock, make sure to turn off 3G on your iPhone, this can be done by navigating to Settings -> General -> Network -> 3Gand disabling the toggle.
Full instructions can be found here.