Sure; there are a couple out there. Lifehacker's was the most straightforward and detailed. The process is much the same as jailbreaking. http://lifehacker.com/5572003/how-to-downgrade-your-iphone-3...

I had some shenanigans with it being an older backup and so had to re-transfer my email accounts from my compy to the phone, but that was just a couple checkboxes in iTunes and setting up MobileMe.