Hmm well I may be a bit late here, but the new model Foreman 500's have a scraper system on the discs. Not too keen on how exactly it works, but basically theres 2 scraper things on each caliper that magically clean the rotors.
Seriously though, it works really good. After riding through a clay pit I looked at em and there was big piles of clay that were built up on the scrapers, I just picked em off, never seen any other kinda mud stick to them though. I have come flying out of mudholes and done stoppies with my brakes.