Frank has never officially retired. He simply wants to direct movies, and that eats up most of his time. As much as people tend to think this is a fairly recent decision, he was moving towards directing more even when Jim was still alive (note a complete lack of new characters played by him on either Fraggle Rock or The Jim Henson Hour). The only difference is he had so much fun working with Jim that he was more inclined to say yes if Jim asked him to do something. Now that the camraderie is gone, he just says yes less and less often these days. Making things worse is when people like Marlon Brando, on the set of Frank's most recent film "The Score," deliberately undermine Frank as a director by calling him "Miss Piggy." You can see why that might make him less than thrilled to return to the roles.
In spite of all this, originally Frank wanted to be the sole performer of his characters because he cared so much. However, his lack of availability led to a very small representation of Fozzie and Piggy and Bert, etc. in the 90s. Audiences stopped connecting with the Muppets as much during this period with these crucial characters missing (among other reasons). That's why in the last couple of years the Henson organization has gotten Frank's permission to start finding new performers for his characters, to increase their profile. In fact, he's even been willing to go in and work as a mentor with them, playing Grover opposite David Rudman as Cookie Monster or playing Cookie Monster opposite Eric Jacobson's Grover, in this most recent season of Sesame Street.
If you need further proof that he hasn't given up on the Muppets entirely, he appeared in a BBC documentary called "I Love the Muppets" this past March; he was interviewed by Muppet writer Craig Shemin for the "Music, Mayhem and More: The Muppet Show 25th Anniversary" CD; and he appeared at a Palisades/Henson party just within the past few weeks celebrating the good sale of the action figures.
About IMDb: Their information is inaccurate. Eric Jacobson performed Piggy, Fozzie and (as far as we know) Animal (maybe even Sam) in the movie. IMDb just grabbed some information from another Muppet production in order to get the cast listing -- in this case, Muppet Treasure Island. At one point, the database said Steve Whitmire performed "Abraham Smollet" in the movie rather than Kermit the Frog! Look for the listings to be updated after we Muppet fans see the movie, check the credits, and then e-mail the database with our reviews and an accurate cast listing.
Hope that helps. Try to get some sleep now.
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I know for certain that in MFS all of Frank's characters were performed by others and he dubbed the lines later.
