Well, according to this rather excellent article over at vintagecomputing.com, there's a reason it now looks like a loaf of moldy bread--and it isn't because you're a filthy pig or a smoker.
According to Benj Edwards, the reason is best summed up by this message from one of Nintendo of America's customer service representatives:
"The Super NES, as well as our other systems, are made with a plastic containing flame-retardant chemicals to meet safety guidelines. Over time, the plastic will age and discolor both because of these chemicals as well as from the normal heat generated from the product or exposure to light. Because of the light color of the plastic of the SNES and NES, this discoloration is more easily seen than with other darker plastics such as on the N64 and the Nintendo GameCube."
Before coming to that conclusion, though, Edwards contacted an expert in the field--namely, Dr. Rudolph D. Deanin, founder of the graduate program in Plastics Engineering at the University of Massachusetts Lowell--and conducted quite a bit of research.
Anyway, if you're like me and you have a yellow SNES sitting in your closet--and you'd like to know more about why it now looks like a loaf of moldy bread--head on over to vintagecomputing.com and read Edwards' article.