We owned a really cheesy second-hand backpack for a while. After we discovered just how much the kids loved 'em we made an investment and bought a much better model made by Kelty in mid 1996 from the REI.
This baby backpack has shoulder straps just like a car seat (but much more comfortable) that hold the child in place. Now I have fallen in a river, climbed up steep wooden ladders, and jumped from rock to rock on damaged trails with a boy on my back and never had a problem before this incident - so I am not really complaining. I thought we had tested it well!
The main difference between the restraint system of this backpack and a car seat (the only difference in safety features, I believe) is the lack of a clasp attaching the two shoulder straps together below the child's neck, assuring that the arms remain in the shoulder straps. It took four years of hiking and a smaller infant to bring this flaw to my attention - but seeing your kid hit the concrete on a short walk is definitely an attention getter. I have no idea how the baby came out of this incident without a scratch - but I just consider us lucky! The baby is just fine, thank heavens!
After some consideration, I took the velcro strap from an arm sling and sewed it onto the shoulder strap to assure it wouldn't slide. The velcro attaches quite well to the fabric of the shoulder straps and we have had no further problems.