not specifically, but you could use jitter with a camera mounted on the ceiling facing down thats using something like cyclops to see if a pixel is filled.. or use the camera vision (CV.Jit) objects to detect how many individual shapes there are.

As far as a sensor though, there is no 1 sensor that would do that, nor can I think of a combination that could. There are a few that would be nice and helpful in that environment, but definately not give you exact number of people in room

rotary encoder+ home-made turnstyle

I kinda like that turnstyle option too, never would have though of that. Kindof like a subway.. that seems like a great ooption if theres only 1 entrance and 1 room. If you have jitter and a cam, I suggest playing with my idea, but the other is pretty good too if not.

also on that entrance style, you could use an infra red diode, check out the infusionsystems lazer +IR, that could be along the floor so as to be discreet, and it should be invisible unless theres smoke.

http://infusionsystems.com/catalog/info_pages.php?pages_id=128