When I first started teaching, way back in 1991, I began the "Look who's talking" project. My wife and I would send out a stack of letters to various celebrities, asking for letters and pictures with words of encouragement for my students, that I could display in my classroom. About 90% were either unanswered or canned responses from fan clubs. About 5% were just autographed pictures. However, the other 5% were actual messages of note, which I display every year in my classroom.
I wrote to James Randi in 1994. Here's his response, which is probably the best I ever received:
http://www.wendingourway.com/celebs/randi.htm
Other submissions, along with some of my commentary can be found at this URL:
http://www.wendingourway.com/lookwhostalking.htm
I have to note that by a strange coincidence, I sent letters to both Isaac Asimov and Carl Sagan the weeks that they died... Sucks that I didn't get off my ass and move faster.