My series 0 SS got one small scratch on screen. My theory was that I scratched it somehow with my diamond wedding band, since that is about the only thing that I own that is hard enough to do it.
My series 2 aluminum only had one small scratch on the screen as well, even though I rarely wore a screen protector.
Based on that experience , I went with the aluminum for my series 3. A week after getting it, I had a large, deep scratch in the screen. I used screen savers after that, but they are kind of a pain to install properly. I took mine off last month, and just decided to deal with it since I knew the upgrade was coming, and it has gotten multiple small scratches since then.
I am looking forward to the sapphire screen, and will be more careful with my wedding band.