As of now, there is only one category of iPhones fully jailbreakable and unlockable on OS 4.0; the rare breed of iPhone 3G’s that are running the bootloader 05.08. With these devices, ifinally unlock the device using ultrasn0w. The purpose of this guide is to show you how you can do that final step.

Please note that this process is theoretical on our end, and while it should work, we haven’t tested it on any of our devices yet.
We would again remind you that this process is for iPhone 3G with bootloader 05.08 only.
On your jailbroken iPhone 3G, use Fuzzyband to downgrade your baseband from 05.13.03 to 04.26.08, running OS 4.0 beta.
Next, start Cydia and add the following source to your list.
http://repo666.ultrasn0w.com
Let Cydia verify the source and complete its automated repository addition process.
Once the addition is complete, search for ultrasn0w and install the application.
That’s it. Technically, all you need to do now is give your iPhone a reboot, and the next time your device should be unlocked.
Please note that this jailbreak is mainly for developers, since the OS 4.0 beta itself is quite buggy at present. Also keep in mind that you must have bootloader version 05.08 for this unlock to work. Proceed with caution. It may be better to wait for a public release.