I bought a blue DSi two days ago and after playing Zelda PH for a couple hours I noticed some decent scratches on it; not the kind you can just rub away. I looked at the stylus under a microscope and noticed a burr around the tip of it, and that was the reason it was being scratched. [By burr I meant a little ridge of jagged plastic around the tip the leaked out during the casting process, the mold was not pressed securely enough together and the burr was not filed away.]
So I returned that DSi and I bought some screen protectors and a crystal case from Dealextreme. The matte finish on the DSi feels like it would absorb a lot of sweat and get discolored, unlike the glossy plastic finish in the DSlite. Once they are delivered, I will buy a new DSi, don some latex gloves and set up a work station with a fan and put on the screen protectors and case.
Yeah, as you can see I'm a whore when it comes to protecting my electronics.