This thread has been bouncing around in my head for the last couple of months. I have built a 'people counter', which although it is most certainly a work in progress, I am heartened with the results so far. I built it using the littleBits Arduino-at-Heart module and supporting circuitry, and a Vernier CO2 sensor. You can see it here, on the littleBits website.
I use a fan in the device for sampling, and to hasten response time - I can tell within a few seconds when a person enters or leaves a medium sized room, with normal, 'classroom' ventilation. I gave up on precise number reporting, and now assign three categories: empty, a couple people, and crowded. I also read out %CO2 (to one significant digit), which in an empty room is about 0.04%, to a crowded room, which is about 0.1% CO2. This narrow range is what limits the precision of the count.
Over the summer, when I have more time, I plan on trying to develop this into a small package using the device I referenced above. I had intended on using an Arduino Micro board for processing, but 'some people' are trying to convince me to use an ATtiny84 or '85 instead.
As (if?) I make progress, I will come back and report here, but I probably won't get very far until June or so. Stay tuned!