Frank Oz was not only the voice of Yoda, but also puppeteert him in Episode 5,6 and 1. In episode 2 and 3 Yoda was complete digital.
In a muppet christmas film, based on the classic "It's a wonderfull life" Yoda has a cameo when an Angel wants to help Kermit to another dimension where he never was.
This muppet show is great, I thought it was availeble a couple of years ago on a dvd box with 12 episodes of the muppetshow, also some show with John Cleese.
The episode has Mark Hamil, who plays both himself and Luke Skywalker, C3po, R2d2 and Chewbacca. The role of Leia is played by Miss Piggy and the role of Darth Vader is played by Gonzo. Han is skipped, as is Obi-wan kenobi (hey, it's the muppet show, did you expect they do it perfect :-P :-D ).
Kermit: Its not easy being green.
Yoda: What you say. *yoda* :-D