We are a company of geeks and nerds and we love it that way. We just are about to finish up a project that is really close to our hearts because it is really the kind of nerd stuff we started out doing. It is a tachometer app that measures RPM via iPhone camera.
There are a lot of details that go into making it work but here is roughly ho it works. It basically takes snapshots of the rotating fan at regular intervals, finds out the center of the circle from those points, determines the angle of rotation per unit time and tells you the RPM on the rotating body.
It works till 200 RPM consistently as of now but we are trying some fancy approaches to make it much faster. I will keep you posted on how that goes.