As promised two days ago, the Ultrasn0w unlocking tool for the iPhone 3GS and iPhone 3G, running iOS 4.2.1 and 4.1, has been released by the iPhone Dev-Team. The unlocker works on basebands higher than 05.13.04. The iPhone 4 has different basebands, thus its carrier unlock will take a little longer to prepare, but it's in the works.
As usual, if you are uncomfortable to tinker with your device, leave it
at that, or, if you want to jailbreak and unlock, do it at your own risk. Your device has to be jailbroken first, so have a look at this video tutorial how to do the tethered jailbreak. Afterwards, watch the video below for unlocking guidance.