On the John Muir Trail I mounted our Powershot G3 on a mini-tripod and slipped that through a D loop on the strap of my backpack. Having the camera so convenient meant that I was able to snap a picture without even stopping, and we decided that on the PCT we wanted the same level of convenience. However, the Digital Rebel is too heavy to use the same system, so we needed to come up with something better.
Enter recycling. We had a plastic bucket that some food item had come in at some point and we decided to turn that into a camera holder. It is not complete, as we might still put some sort of padding around the top rim. In its current incarnation, it weighs just under 2 ounces. All we did was cut two slots in the side, through which I feed my hip belt. I am still able to adjust the waist strap with the holder on. The camera goes, lens down, into the bucket and the lens hood prevents it from hitting the bottom. I clip the neck strap (which will likely be replaced by a much lighter one) through my chest strap on my pack so if the camera holder fails, our camera won't crash to the ground.
Here are some pictures of the camera holder in use: