Ok, hold on guys.. you are getting to excited. This method will only help iPhone 3G users which are currently on iPhone OS 3.1.3 with 05.12.01 baseband and bootloader 05.08. This will not work with iPhone 3Gs.
So if you are think you fall under the rules described above, all you need to do is to download Fuzzyband from Cydia which was updated to work with the latest 3.1.3 firmware.
One you downloaded you need to press the only button available on the screen that says “Downgrade” . When the process is complete you will be able to unlock your device using ultrasn0w.
If you have no idea what bootloader version you got and if Fuzzyband will work for you, just run the app and it will tell you. You either can downgrade or not.