I run a Greyhawk campaign and monks have been a part of it since 1e AD&D. Consider the Scarlet Brotherhood, for instance.
My current game has one monk character. I designed a monastery for him called "The Order of the Crescent Moon." For my game, this is the order from which the ninjas of the crescent moon separated years ago. Hence, the two are bitter enemies. My player's monk is, of course, a member of The Order of the Crescent Moon. This builds some interesting adventure possibilities into the game.
At any rate, I am slowly working on my own campaign world, and I have not really thought too much about whether it will have monks or not. I will probably work them in some how, even if they are extremely rare or only come from an exotic location. Perhaps the elves or some sect of elves have the secret of the martial arts and will not teach it to others. PC monks would have to be elves.