This is very interesting. A friend of mine is trying to make an Arduino quadcopter so he can learn the framework. It's already functional, but there's still some work to do like stabilization algorithms and GPS stuff.
I would characterize their main efforts as building a modular autopilot software & hardware infrastructure, for which the optical flow sensor is one part. They are both an academic research group & generous providers of an open source platform competitive with the best out there.
