One idea is to let the user pick whether or not to back stuff up.I know for sure that I have and will save things that I could care less about losing.Just and idea.
I don't think I'd let users be selective. What I'm aiming for is something that is totally transparent to the user and happens without them knowing/having to do anything. Of course I could include an option to disable it entirely if it was giving people trouble or if they simply didn't want it.