Here's one of a series of recent shots I made with a special rack to mount a 35mm SLR to my mountain bike.
To get this shot I made an extended baseplate that attaches to an existing standard bike rack. There are lots of ways to do this, this is just one of them. I used two pieces of 3/8" aluminum rod carefully bending these and bolting them to a piece of 3/16" thick flat aluminum. The aluminum plate has a hole in the center that allows bolting a heavy duty ball-head to this rack. I use a remote cord to make the shot while I'm riding. It's stiff vertically but I used a wide nylon strap to synch this up (from the seat stay crossbar to the ball head) to give it more support horizontally with this (heavy) Nikon F4s. Next time I'd probably make the main strut run horizontally. But this config works very well. I also counter weight it with a sand bag attached to the opposite side of the rack. Works great with video cams as well.
Mark