The 20D center AF point is a cross type sensor that is active when a lens with the max aperture f/5.6 is detected. When a lens whose maximum aperture is f/2.8 or faster is detected, the 20D switches on the high precision horizontal sensor array which is sensitive to vertical lines. Please see my append in dpview 20D forum for more detail.
http://forums.dpreview.com/forums/read.asp?forum=1019&message=18215638
