I use a OP Tech Bino/Cam Harness with a small Tamarac case for my Canon G9. It would also work for a DSLR if you find the appropriate holster. The harness goes on 1st, under the backpack.

I checked out the Think Tank website suggested earlier. They have a much beefier version of the same thing as well as holsters for DSLR's Think Tank Digital Holster

I think the important thing is to have instant access to you camera.
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Just because you don't take an interest in politics doesn't mean politics won't take an interest in you.... Pericles (430 B.C)