Mickey Mouse Meets the Air Pirates is a fairly good example of fair use pushed a bit too far. The court ruling said that "some" resemblance was fair, but the characters in the comics were actually presented as the actual Mickey and Minnie, dropping their drawers for quickie sex and generally behaving in a rather non-Disney fashion. It does lead one to wonder what "some" means, though. Probably depends a bit on how the judge is feeling.
http://en.wikipedia.org/wiki/Air_Pirates