Zero litter at the event, but yeah, some people are hella irresponsible w/ disposing of the trash after they've left the event. They're not just throwing it on the ground (that I've seen, anyway), but they're leaving it in any random trash can they can find, rather than paying to dump it (which there is ample opportunity to do -- a cottage industry springs up of people taking trash for $5 a bag post BM).

Probably burning man should just include end-of-event trash disposal in the ticket price, and have an official collection area people can dump their trash. (Probably some reluctance on BM part to do this 'cuz of the somewhat-inaccurate "radical self-reliance" principle.)