Jim Korkis sent me this link to a pretty good article about the movie parodies of MAD magazine. In case you don't have time to read the entire piece, I'll quote the last paragraph, which is especially good…
MAD has never been successfully sued, but that hasn't stopped people from trying. The magazine once received a letter from Lucasfilm's legal department after their Empire Strikes Back parody, demanding that they recall all printed copies of the issue and destroy them. MAD replied by sending a copy of another letter they had received the previous month — from George Lucas, offering to buy the original artwork for the Empire parody and comparing Mort Drucker to Leonardo Da Vinci and the parody's writer, Dick DeBartolo, to Mark Twain. They never heard from Lucasfilm's legal department again.