I use the Contour HD camera, and I've got 3 of them. I don't run all 3 at once, I have that many for race filming. But I do have a setup where I can run 2 on the bike; one facing forward, and the other mounted about bumper level on my traffic side pannier, facing rear to catch plates of vehicles as they pass.

As for bike vs. helmet, I find that helmet mounting is better. On the bike, your camera points where your bars point rather than where your head does. I like the ability to aim the camera to my view while travelling in a different direction, for instance, if you hear a tire screech or a car horn and you turn to look; the camera follows along.