I guess it depends on your definition of scratch. I'd say it's physically impossible not to have abrasive material on your fingertips for much of those 10 months - no matter how clean the environment. The N900 screen is plastic, the dust and gunk you get on your fingers can be as tough as diamond - put those two together and you WILL have scratches on your screen, even if by your definition you don't.

If you want your screen to remain scratch free, then buy a screen guard.