The iPhone Dev Team has released an unlock for iOS 4 on the iPhone 3G and the iPhone 3GS, with baseband 04.26.08, 05.11.07, 05.12.01 and 05.13.04. Before you can unlock you iPhone you need to jailbreak it first using redsn0w-beta 0.9.5b5-3. After you jailbreak your device, go to Cydia and add the repo repo666.ultrasn0w.com and search for ultrasn0w (version 0.93). Install it and reboot your device.