From TV Guide:
Pop sensation Ashanti plays a very hip Dorothy as the Muppets go over the rainbow. This time around, Dorothy hopes the Wizard (Jeffrey Tambor) can make her a superstar. Kermit the Frog is the Scarecrow, Gonzo is the "Tin Thing", Fozzie Bear is the Lion, and Miss Piggy is both Glinda and a leather-clad, motorcycle-riding Wicked Witch.
There is something very wrong about Miss Piggy in black leather. Though I'm not sure how much wronger the Muppets can *get* after A Very Merry Muppet Christmas.
The accompanying picture has everyone described above except Tambor, and also has Rizzo and Pepe - I think Rizzo is supposed to be a Munchkin. I don't know what Pepe is supposed to be.
Friday, May 20. Mark your calendars!
EDIT: And from IMDB - Quentin Tarantino as himself.
I... have nothing to add here.