I DO NOT take any credit for this.
I read this on another forum and was written by a user called delldude1989 who found this solution. The solution to downgrade from iPhone 3.0 to iPhone 2.2.1 is as follows:

1. Attempt a restore of 2.2.1 in DFU Mode

2. Get the iTunes error message.

3. Reset device

4. Launch QuickPwn

5. Press next and get past the first 3 steps.

6. Bam, phone is fixed.

I have not tested this myself know let me know if it works or not

Thanks, it's work but Modem Firmware now 04.20.01

Cheers

Thank you for testing it and confirming it works.
Can you let me know if all features work in 2.2.1 with the 04.20.01 baseband?
SMS, 3G etc

Yes everything work normal like never upgrade! My iPhone 3G is Official unlocked version.

Guys, I've read the 3.0 release notes but they aren't clear to me.... are you able to use this beta firmware fully without having a dev account or does it need some sort of dev activation? I'm considering purchasing the SDK to get it early if it does.

Hi
Im not sure that updating to 3.0 & ending up on 04.20.01 BB was or is a wise thing to do, especially if you wanted to unlock on a non official network in the future. Because I wouldn't think its posable to downgrade the BB backwards

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It's just accident but lucky mine one is officially unlocked then no problem. Anyway it is not recommended for iPhone using yellowsnow to unlocked.

Cheers

Another Q, how do you extract the DMG in Windows?

Edit: with TransMac, that's fuckin how

No joy here. Quickpwn does not even detect the phone.

Cant get it out of DFU mode either. Seems like I'm screwed!

Thank you it works like a charm!

Guys dont give up keep on trying it will work eventually i had the same issues and i kept trying and it worked