I don't know about something called a 'one-shoulder backpack', but our AAA magazine is advertizing an ergonomically designed back-pack that looks like it has only one strap. Apparantly, it's designed to fit better than a traditional frameless backpack, therefore causing less stress than 'normal' packs.
I'd thought about picking one up, but haven't gotten to the point where I'd be willing to cough up the $69 or so.
-Joe