When I take my DSLR (or any of my cameras, really) on the bike, it's in my pack. I don't want it taking the abuse/shock of a dirt bike.
On the snowmobile, I've got a hard case on the tunnel I keep my Sony
Re-rigged the setup on my new sled, got a bit more vibration reduction to the case, but i'm still probably going to carry the Nikon in my pack and leave the Sony in the case.
Biggest thing is a camera that's easy to get to. If it takes 5mins to stop, pull the pack off, dig the camera out, turn it on, dial in the settings, and get the shot... you're gonna miss a lot of shots.
I roll with a Contour on my helmet for the "Oh-Shit!" shots, and the Sony in the case is pretty quick to get at.