Marc Böhlen of Real Tech Support has an interesting take on surveilling people in a building - instead of mounting surveillance cameras up high and watching faces, he mounts them down low and images feet and shoes instead. The idea is to have a surveillance system for automatic people counting and safety that is less invasive of people's privacy. For example, security personnel could use it to know if there are any people in a particular area of a building in case of emergency.
As a concept, I think it is intriguing. Thinking about it from a systems perspective, however, the efficiency of emergency response would be improved by knowing what a person (not just their shoes) looked like as well as being able to more thoroughly surveil the area. Putting both systems side-by-side would allow one to leave the more invasive system inactive until needed, but that would be more expensive. What it seems to come down to is a classic tradeoff between privacy, security, and cost.
A link to his site is here.