Aubie:

I can't *prove* it, but I am convinced that Continental did this once on a flight from St. Louis to Houston. They called a maintenance delay; but when the problem was "fixed," Barbara Bush got on the flight. I'm certain that the delay was for the Secret Service people to check out the plane.

Again, there is no absolute proof that my conjecture is correct, but if not, it's one heck of a coincidence...