buy a d40x and hang it on your neck for 4 months straight without taking it off - your neck won't feel any pain at all.
1) Neck sling the camera as usual
2) Clip a carabiner to the sling (position at the back of your neck)
3) Wear your backpack as usual
3) Attach the carabiner to the handle of your backpack.
The weight of the camera and backpack should offset each other and be distributed on your shoulders, not the neck.
Hope the description makes sense.