Thsi is kinda was has prevented me trying to go back to 3. I don't have a "backup" pre iOS4, that I'm aware of. I don't think it gives you an option to select which backup you want (timewise); liek if you were doing aa windows restore or soemthing.
That and they said the little add on you need (to exit recorvery mode) doesn't shake well with windows.
But my wifes 3g is CRAWLING on OS4.