That's cool! A had similar idea some time ago, but instead of using the pixel brightness it analyzes the edges in the image and tries to rebuild as mesh, giving a funny "Tron" effect.
It's written in Processing [1] and there's a sample video [2].
This is awesome! Just been playing my guitar with it. When the strings are vibrating you can see spikes where they cross over the frets. What's great is that when you change chords the pattern of the spikes changes.
This is pretty cool, but seems to be a bit broken under linux. At least in my instance the camera was not reading correctly, was getting green and pink lines.
This is great fun, especially if you have a torch you can shine on your face.