I finally tried the lens extender "hack" to enable autofocus with the 100-400 lens.  I covered three of the electrical contacts on my Canon 100-400 lens and attached the lens to the Canon 1.4x extender.  Covering the contacts fools the camera by telling the camera the extender is not there.  The autofocus did indeed work (on the 50D body) but was really slow.  I found that focusing on something at 100mm and then zooming to 400mm while still holding the focusing button part way down allowed faster focusing than zooming to 400mm and then beginning to focus.  A few of the photos were really good, but most were a bit blurry.