Does a hybrid pannier / backpack exist?
Yes. I own a north st bags convertible pannier and think it works great.  It hangs a bit low, so don't try and use it on low rider racks.

http://northstbags.com/products/woodward



Also see Richard Jones Convertible Backpack

http://www.convertiblebackpacks.us/



Also try WOHO bags "NINJA NINJA" convertible backpack:

http://www.wohobike.com/product.php?linkid=133