Sensing busy-ness of a restaurant? I'm planning on an Arduino project for a restaurant I work at that would require determining how busy it is at any point in time, but I'm not sure how I would go about procuring this information.

I don't have access to any of their IT systems or I would have simply pulled the data from their till systems.

I was thinking perhaps measuring the ambient sound of the restaurant as it gets quite loud during peak time but that could be thrown off when the music volume is adjusted.
There are a few ways to  go about it but my initial thoughts would be to detect people entering (and exciting) the front door and simply keep a running total.

Motion detector 
http://www.adafruit.com/products/189

IR beam detectors
http://arduino-info.wikispaces.com/InfraredBeamPair