I had this problem on Mandrake 10, there is a thread on this in the Mandrake forums. I have a similar problem on PCLinuxOS now. It doesn't reset the settings on every boot, it's more like every other or so. Very weird, no solution that I know of.
One tip though: for browser settings, it helped to put an entry in the KDE control center "Components -> Component Chooser -> Web Browser". It seems this takes precedence over the file association settings.