Now this looks really fun...if not a bit dangerous. They're images of hotel guests from the Ormond Hotel in Ormond Beach, Florida taken in 1903 (the images were found here). Unfortunately, the hotel--which was on the list of the US National Register of Historic Places was torn down to make room for beachfront condominiums. I love the image on the lower left (click it for a larger view), both of the men have their feet off their pedals and are being pushed by the wind...what could be more fun and freeing.
But a sail on a bike? Have ever tried wind surfing? Really hard to ballance!
I would only try this on a bike with a mechanism which allows me to get rid of the sail quick, when I want to. ;-)