Mythbusters showed soda cans, polished with chocolate, being used as a firestarter. Also, an ice lens used like a magnifying glass. However, while I am not going to start littering, I am not going to much worry about pop cans and ice starting major forest fires.

I am more worried about off road vehicles in dry brush, irresponsible campers with fires, people burning garbage in 55 gallon drums in the summer during a drought, careless smokers tossing butts out windows, and kids playing with matches.