My screen was perfectly fine if you exclude the colour going spastic. But occasionally my stylus seemed to scratch at it a bit, which I can only assume was the result of dust/dirt on the screen or the stylus. Or perhaps as Iceglow said, the tip was sharpened a little.

Taking care of a DS isn't difficult. If you let it get super dirty and then press too hard when you're playing gimmicky games then of course you will have a ruined screen. Tossing them around also isn't good for it either.