They don't have a US distributor anymore that I know of, but Aarn Packs makes the perfect packs for quick-drawing a camera. They basically have a pair of modular bags that are part of the main shoulder straps (but have stays that distribute their weight to the hip belt). They have a couple of camera-specific models. I've carried a 50D plus a 70-300L, a 10-22, a 100L, and accessories with it all immediately accessible on the front of my body (the rest of my gear goes in the traditional portion of the pack). Aarn packs include water-proof bags for everything as well.