That "screen protector" that comes with the N800 is not very optically clear and does scratch very easily. I have also been using the N800 daily and very often with both my fingers and stylus and at the end of the day when I clean the screen I have zero scratches and marks on mine.

I think Nokia thought ahead and figured how to design the screen to prevent markings from normal use.

In fact I asked this very question at CES and they said they test that by putting products into a tumbler with pebbles and other things to see how the case/screen will fair with serious abuse.