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