I had the same problem (inadvertently upgraded with an unlocked iPhone.
So I found
iClarified - iPhone - How to Downgrade Your 2.0 iPhone to 1.1.4
This helps quite well but I got stuck at the end and had to restore firmware with iLiberty and then unlock/activate with Ziphone.
I have lost a few days' data from the last sync on this computer but the main one I use is a Mac with all my up to date stuff. I will have to downgrade iTunes to 7.6 then re-sync.
The positives are that I have deleted a whole host of apps I had downloaded for fun and just haven't used!