I'm actually quite amazed that a forum full of Magician's didn't see this one coming. Wayne Dobson used to open his show with this all the time as a sight gag. He of course had the advantage of being able to hold his finger to his temple and close his eyes in order to convey the message. This was part of a quick string of gags he would use, as seen here. http://www.youtube.com/watch?v=qAWi15MAOmo&sns=em" target="_blank
I would say that Isaac Asimov once wrote an SF story for telepaths. It read, 'Aw, you know what I mean' or words to that effect! As for the joke, tut tut, naughty naughty.....