I've experimented with a few setups, and I've got the following conclusion for riding with a DSLR:
Handlebar bag: Great for stopping and taking out your camera, if you shoot while riding, you'll end up leaving it open, one good bump later you're camera flies out of the bag. I used this solution when I shot the ms150 last year and it worked alright, but I nearly lost my camera a few times.
Slung over the shoulder: Using a standard canvas/vinyl strap over your shooting arm will have a tendency to fall in front of you if you ride anything close to prone. If you sling it over the opposing shoulder, it helps a little in keeping it at your back, but a couple bumps later it'll be dangling right in front of you again. If you can get a neoprene rubber strap this works out pretty well if you wear it a little tighter than you normally would - your camera will dangle at your chest and be pretty safe, but within reach. Problems with this setup are that the camera is left out in the elements, and if you need full range of motion shooting, you'll have to un-sling and sling it over and over again.
Hip pack: Great solution, keeps the camera safe, easy to access, but while shooting you're left one handed and the camera is un-tethered, which could lead to the end of your camera.
My newest setup will be a Black Rapid RS-7 strap combined with a very simple hip-pack that holds my camera in a holstered, not packed positon. This keeps the camera safe when it needs to be, but easy to whip out, as well as keeping it tethered, so the camera wont fall to the ground, and I can drop and reach for the handbar in an instant if I need to.
Some other things photogs may want to consider, is that if you're shooting with a more mechanical/older camera, you really shouldn't mount your camera to your bike as it can put a beating on older components (modern slrs/point and shoots can handle a good bouncing a lot better), jarring lens mounts, aperture leaves, levers, etc.