DIY Photography has a very nifty little project up on how to convert a single-camera bag to make it more bike friendly. Put together by Pierre-Yves Chopin, all it requires is a strap, velcro, elastic, hooks, and a bit of a dab hand at sewing. Chopin took his Lowepro Toploader zoom 50 AW, and added a horizontal strap to the back of the bag. Two velcro strips were looped around this strap to attach to the handlebars, and then the elastic was used to attach at the base.
The hardest part about this whole tutorial is sewing the horizontal strap in place well — everything else should only take a few minutes to whip together. That way, you can take your camera with you when you're out biking, and not worry about it banging around on your back — just be careful if you end up plowing nose first into anything, your camera gear's now on the line.