Item frames have disappeared two different times in the same world after a save and reload of the world.  Both times have occured after a complete shut down of the game.  Other than the items contained within the frames nothing else has disappeared.  Though the frames were near a chunk border they were at least a block away but not more than six blocks away as I saw in the second occurance.  The first instance all item frames one block away from the border disappeared.  In the second instance item frames 2,3 and 4 blocks away disappeared, but the item frames 6 blocks away remained.