Daffyfan-
All the Time-Life first season eps use second season openings. Vincent Price and Rita Moreno use the Bob Hope opening, Mummenschanz uses the Bernadette Peters opening, I think Juliet Prowse uses the Milton Berle opening (I would have to double-check, I don't watch it now that I have the original).
The response I got from Henson.com as to why this is done is because the original first season ones were low-tech and low-budget and Jim himself preferred the later ones, so nowadays they're substituted. They were run on Dutch TV not too long ago. Only place to find them these days is in private collections.
And to add to Jog's list:
#51 Roy Clark-
Scooter: "Somebody kill that light!" (gunshot - light falls)
#52 Gilda Radner-
Gonzo: "I always get butterflies before a show." Butterfly: "Not this time." *SMASH*
#53 Pearl Bailey-
Scooter: "Everybody on stage! Everybody is on stage...!"
#59 Raquel Welch-
Beauregard pulls on a rope and a sandbag falls on his head.
#60 James Coco-
Beauregard: "Here's your script - WHOOPS!" *trips*
#71 Lynn Redgrave-
Beauregard and some merry men stand in the wings and an arrow flies toward them and hits the wall.
Beauregard: "Say, that was GOOD!"
#76 Dyan Cannon-
Beauregard, watching Foofoo: "Who let this dog in here?"
#87 Anne Murray-
Floyd plays a bass solo and a gorilla smashes him between two cymbals
#92 Alan Arkin
During Rowlf's piano solo, Beauregard starts sweeping on the piano
#95 Carol Channing
A feather boa (see Twiggy ep. opening #, one of the backups) comes out of Zoot's sax
#96 Diana Ross
A rattlesnake goes into Zoot's sax
David "Gorgon Heap" Ebersole