Tout Terrain, a German Bicycle manufacturer, makes a rack to hold a backpack. It attaches to the seat post. The back pack they sell is called the Sherpa. The rack is called the Sherpa Support.
Read about it.
You can buy this set up from Peter White cycles. The backpack is $134.00 and the holder is $106.00.
Anyone know of a more inexpensive but similar solution? Parts to make your own?