I disagree. Autopilot controls are not sufficiently precise for an inexperienced pilot to hit as relatively small a target as the World Trade Centre or the Pentagon without precision guidance. It is conceivabe that a GPS-linked FMS guiding the autopilot might manage to hit these buildings, but that would require detailed knowledge of the precise FMS fitted to each of these aircraft. It is far more likely that these terrorists used experienced light-aircraft pilots who received a small amount of training in a B757/767 simulator to practise these exact manouevres.
It's a sick excercise, I know, but if you try to achieve something similar in FS2000 or other computer flight sim using an autopilot, I think you will discover that the chances of 'success' are far too small for the terrorists to have accepted. These pilots knew what they were doing.