This is a non-trivial question, unfortunately. Nokia's built in backup application really only backs up the built in stuff, although it does back up your repository list to make reinstalling your apps a bit easier. I keep the debs for all my commonly used programs & libraries on one of my SD cards and tar up some of the other stuff that I would loose if I were to reflash. Nokia really needs to use proper package management for the OS itself and make reflashing an option of last resort. Releasing new versions as a embedded image only is a really frustrating aspect of Maemo. The other thing I hate has been that Nokia has often only provided very general/vague information about what bugs got fixed and what features have been added in a given release. I have upgraded my 770 & n800 on more than one occasion only to be left feeling like "why did I waste an hour and a half of my life doing this again?"