My friend and I had an argument in my truck about how they got Mr. Ed to talk. She insisted it was peanut butter, but I've read that that is just a rumor. My old school had a book about horses in Hollywood, and it said to notice that whenever Mr. Ed talked he had on his halter. Apparently they had put pressure on him through his halter which got him to open his mouth. Knowing this, I was watching an episode and noticed that the halter is always a bit snug around his bridlepath and ears.
Have you heard any other rumors? Peanutbutter is the only one I know of.