As a fellow photography nerd, I feel for you. I searched on this for a while and settled on the Lowepro Mini Trekker backpack. I figured a backpack would distribute the weight on my shoulders well and offer the capacity I wanted. I've been able to fit the following into mine: Canon 20D with 200mm f/2.8L attached, Canon 85mm f/1.8, Sigma 50mm f/2.8 Macro, Voigtlander R2 rangefinder body with Konica 50mm f/2 attached, Leica 35mm f/2 M-mount lens, Rolleiflex f/2.8D TLR camera, Canon 580EX flash, Canon 1.4X telephoto extender, 2 quick release tripod mounts, many lens filters, Polaris light meter, misc tools and attachments for the above, misc rolls of film.

A backpack works well for a photo bag because it carries a lot. The Lowepro is nice because it has configurable velcro-configured compartments for everything. A backpack is somewhat inconvenient as a photo bag because you have to take it off, put it down, open it up, and sort through it to find what you want every time you want to change equipment. That said, I wouldn't think twice about buying one again. Give it a look.