I was just thinking, with my N-Gage not booting up, that wouldn't it be possible to have a drive (such as the ROM Z:) with all the system files backed up, and that the phone could recognize if it can't boot properly and prompt the user if he/she wants to restore original files, instead of having to take the phone to a Nokia Service Center or something.

Also backuping contacts, notes and calendar entries to either MMC or other drive should be a pre-installed function IMO.

Backing up contacts, notes, calendar entries, settings, etc., to MMC *IS* a pre-installed function. Look in the Options menu of the "Memory" app.

If you want more control than that, try, e.g., this:
http://www.spritesoftware.com/products/sprite_backup_symbian.html

You can also use the Nokia PC Suite application and its backup function to make a backup of the phone contents to your PC.