Well I think we all know the king of cameos in the film industry, so with that said my vote goes to every cameo in It's a mad, mad, mad mad world. That bit where Jerry Lewis ran over the guys hat, or when those two blokes crashed the plane, the fire department showed up with firemen Moe, Larry and Curly.
But in terms of movies that weren't made half a century ago I'd have to say seeing Leonard Nimoy in the recent Star Trek film was a real highlight. Oh and I've heard that Steve Urkel made an appearance on an episode of family matters.
EDIT: Oh and to the OPs credit Back to School is a hilarious film. It was the first one I downloaded off the new Zune service on xbox live and I highly reccomend it.