Jackie Mason, who voiced Rabbi Hymen Krustofsky, is apparently going to be making an appearance in the Spurlock special.  At least, according to In Entertainment:

The Simpsons are set to mark their 20th birthday celebration with an hour long special, including the reprise of Krusty the Clown’s rabbi father, played by Jackie Mason.

The special, will start off with a documentary entitled “The Simpsons 20th Anniversary Special: In 3D on Ice, which will be hosted and presented by Morgan Spurlock, the filmmaker behind ‘Super Size Me’.

That also makes it sound like the documentary he’s been all over the place filming will be only a part of the hour long show.  Are we in for a “138th Episode Spectacular” style special with celebrity guests reminiscing about the show?  Meh.