*In Woody Page voice: "Why do I always have to straighten you guys out?!?"
Nah, just thought I'd come in and help.
Jaap, here's the info on the three versions of Muppets Wizard of Oz.
1 In the animated series Muppet Babies, check out the episode entitled By The Book. There, Piggy was Dorothy and Rowlf was Toto, Kermit was the Scarecrow, Fozzie was the Cowardly Lion, and Gonzo was the Tin Weirdo. Scooter was the Silly Sorceror of the South, and Bunsen and Beaker were the Wizards.
Of course, this was all in their animated baby forms.
2 As for the live version... Piggy was Dorothy, Scooter was the Scarecrow, Gonzo was the Tin Weirdo and Fozzie was the Cowardly Lion. Though Rowlf wasn't Toto for this sequence, he did come in during the black and white portion/opening of the medley. This was part of the TV special The Muppets Go To the Movies.
3 And of course there's the soon-to-be-released TV movie version of Muppets Wizard of Oz which we're looking forward to in May 2005.
Hope this helps and have a good weekend.