http://www.vintagecomputing.com/index.php/archives/189
"Why the SNES changes colors"
It says that it's a combination of UV light, heat, and oxygen in the air that causes a reaction in polypropylene, impact styrene, and/or ABS plastics.
I have a sticker on the bottom of my SNES that says its frame is made of ABS.
CASE CLOSED.