I've started combining photography and cycling this year myself. My solution for my camera (Canon 400D) was a Lowepro bag. Nova 160 AW model. It has flat loops on the back where one could strap it to something though it's a bit big to slide it onto a belt and walk around with it on your hip. The extra pockets are perfect for cellphone or whatever other little items you want to carry as well as purpose made slots for batteries and memory cards. This model bag is perfect for my camera and a 70-300 mm lens. It also comes with a rain cover though I've not tested it yet. When I want to walk around away from my bike, it unclips in seconds and I sling it over my shoulder and go with all my important items right there.

Of course, my ride is a bit unconventional. A Trice QNT '06 which is a recumbent trike. So, this solution was perfect for me since I don't have handlebars to use a handlebar bag. The with extra clips, the camera bag fastens right onto the straps of the seat's side pod bags and hangs straight. It sits right at my elbow and I can pull the camera out quick enough I managed to catch a shot of a pair of deer racing across a field. Not a GOOD picture, but I still managed to catch 'em before they made the trees.

With the strapping on the back, you might be able to figure out a quick release way to fasten it to your handlebars for moving away from the bike for a better angle.