Frank Sinatra did in fact cover the song. He was good friends with the song's writer (the late Joe Raposo), who did most of the music for Sesame Street for a good deal of the show's early years.
I think the song is in rotation on MuppetCentral radio, though I don't listen to it, so I wouldn't really know. (heh, sorry.) I think that it might be easily found on file sharing programs such as Kazaa and WinMX.
Frank Sinatra did not ever guest star on The Muppet Show, nor did Adam West (which is a shame, because I think that would definitely be an awesome episode.
) If you're interested, our own scarecroe has put together a really nifty Muppet Show "Cheat Sheet". That should help you out perfectly by way of guest stars. That link is:
http://muppets.scarecroe.com/muppetshow/
Also, for a good indepth Muppet Show guide, check out
www.Kermitage.com .
(Come to think of it, no one with a name starting with the letter "F" has ever starred on The Muppet Show...weird, ay?
)
-Matt