DealExtreme has a camera with a bike mount that shoots acceptable quality video and is quite small, for $20.
http://www.dealextreme.com/details.dx/sku.32022
You have to add your own microsd card. 2gb will shoot for about 2 hours. The battery is also good for about 2 hours on a charge.
Here's a sample video shot from it:
http://www.youtube.com/user/demjp8Rq.../5/WR6P7O9Cs0I
Youtube reduced the resolution, it actually is a bit more clear than that.