I'm sure someone has done this. After fighting the wind the other day, it seemed to me that there is a market for a sail-driven bike. The sail could be mounted on a post behind the rider, and be turned to take advantage of the prevaling wind. With tacking, it could be used even when the wind is coming from a frontal angle. Seems to me that with just a small boost speeds of 30mph could be common (yeah, I know -- for some of you they already are).
Does anyone know of such an invention?