If that sounds like enough cameos, I've barely started. The Muppet Movie also has appearances from Dom DeLuise, Madeline Kahn and Telly Savalas, Paul Williams, Cloris Leachman, Bob Hope, Milton Berle, Edgar Bergen (and Charlie McCarthy), Elliot Gould and Big Bird. Director James Frawley supposedly appears onscreen briefly, but I'm not sure I'd recognize him. I've included two YouTube clips of cameos after the jump: Richard Pryor as a balloon salesman who gets Gonzo in a lot of trouble; and Steve Martin as -- what else? -- a rude waiter.
The Muppet Movie sometimes gets a "cameo for cameo's sake" feeling after you've seen two or three of them that have little to do with the plot and aren't that funny. As the years go by, however, I do get a warm sentimental feeling from watching various performers who are no longer with us -- including, of course, Jim Henson himself as Kermit the Frog.