One comment I received on a previous blog about backpacks was about a backpack literally breaking your back. I decided to do some research on the topic. It appears that a vertebrae in your spine can withstand about 500 lbs of force, which is about 2225 N of force. In order to do some damage to your spine, you would need to have a backpack weigh this much in newtons. This means that your backpack would have to have a mass of 227 kg, or almost 500 lbs to do real damage to your back.
To put this in perspective, I estimated that a relatively full 2 inch binder is about 5 pounds on average. This would mean the backpack would have to hold about 100 binders to do damage.
No only is it near impossible to fit that many binders in a backpack, and so you would have to have one specially made, but you would also have to have a group of people or even a small machine help you lift it onto your back in order to actually do the damage.
Long story short, I don't think a backpack will be breaking your back any time soon.