A question just popped into my mind for which I have no answer at the moment. We address adult men, in formal occasions, as "Mister," unless they hold a doctorate, in which case we call them "Doctor." Americans like to address their President as "Mister President." However, what if he held a doctorate? Would he be "Doctor President"? Has there ever been an occasion where a choice between these forms of address had to be made?
Update: Well, Bill Clinton has a doctorate in Law. I guess "Doctor President" isn't going to happen. Unless an M.D. were elected President, perhaps?