I think that's a great idea and the typical program PBS would be attracted to. And what Sesame Street deserves as an anniversary special, rather than just focusing on the newer stuff.
Well, i think that SS needs 2 kinds of specials... an indepth retrospective for the fans (and I just don't mean seeing the same old clips a million more times. Rare, behind the scenes stuff and how the show got sold and operated for that long), and a big story for kids and parents to enjoy. One on PBS, one on another ntwork or something like that... or maybe just one DTV.
I wanna see what makes the show work on an adult level, while still keeping the illusion of the show... and something not dominated by celebs saying "Oh, look! We were on Sesame Street once."