The Apple iPod nano may be quite strong, but according to many users the display of the nano scratches way to easily.
Actually, not so for owners we've heard from. "I found that my black 4GB Nano scratched within minutes after peeling off the protective wrapper and wiping it with a cotton T-shirt. I put it in a pocket just once and it was inside the soft case that came with my third-gen iPod," comments poster number 188 in that monster Apple thread.
Looks like iPod nano users better leave the sticky plastic over the screen if then want to prevent an overload of scratches on their screen. More info at The Register