Mumblez wrote: "I always thought it would be great to take an ultralight waterproof roll-top stuff sack and attach a nice sturdy harness to it. The most I might need besides the one large compartment would be two water bottle pockets on the side and maybe a small outside pocket for essentials. I really want a pack that can handle 30-40 lbs. but still weigh 3 lbs. or less. I think this could be done if they keep the main bag simple, focusing most of the weight of the bag in the suspension. What do you think?"

I think you've just described Granite Gear's Vapor Trail and Nimbus Ozone packs, and maybe a few others in their Ultralight line. I'm sure other posters will tell you about Gossamer Gear, SMD, and other manufacturers that fit your 30-40 pound, under 3 pound criteria.