If you go this route, look for a backpack that has two entry points. A vast majority of hiking backpacks are top-loading only. This means that if you use a camera insert, it will either be at the bottom where you'd need to rummage through whatever is on top (clothing, snacks, etc) just to retrieve a lens or any other piece of photo gear. Likewise, if you put your necessities on the bottom and camera insert at the top, you'd have to try and fish out whatever item you'd need as well. Best to look for a pack with top/bottom or top/full side entry points. There's also the issue of placing a bladder inside the pack along with your camera gear. Judging by what you'd like to carry, I'd guess that you'd be looking at a pack in the 30-35 liter range...a pack of this size should meet your needs.
I've been using a Lowepro Rover AW II pack for a few years now. As far as "camera" backpacks go, its relatively light (a hair over 3 pounds), has two separate compartments for camera gear (bottom) and necessities (top). Excellent tripod carrier, external water bottle pockets, built-in rain cover, and a decent harness system...really works well for short or long day hikes. There's also a waterproof version, Dryzone Rover. Camera compartment is fully waterproof as opposed to just being water resistant. This one's a bit heavier (about 5 pounds), and designed with a bladder pocket in the upper compartment. Not sure if the Rover AW II is too large for you, though.
I just picked up a Deuter internal frame trail pack at REI in Dallas a few weeks back and attempting to modify it into a camera backpack (has two entry points, top and bottom). So far, the modification looks good. Once I get the tripod mod figured out, I'll post a few pics (I prefer the tripod down the center of the pack, which is taking some figuring out).