The thing that Marilyn vos Savant missed here is that it makes a difference whether (a) Monty
always opens a second door after your guess and offers you the chance to switch, or (b) Monty
chooses whether or not to offer this option. This is not clearly specified in the problem, either as Joe put it, or as MvS did. MvS implicitly assumed case (a), and answered correctly for that case. But some of her readers were assuming case (b), which is far more complex and ultimately unresolvable, and MvS was unable to detect the difference in their arguments - rather than listening long enough to understand, she assumed that everyone who wrote her was just doing it the same incorrect way. Many were, but still...
It's worth noting that the real-life Monty Hall did in fact choose whether or not to open one of the doors, and employed a variety of other mindgames to steer the contestant away from the correct choice, whatever it was. (Or did he steer them
towards the correct choice sometimes, for better ratings?) So you could never really be sure
what rules he was actually operating under.
[This message has been edited by Jim Yingst (edited November 05, 2001).]