Canon have got a patent on lenses with an aperture ring, apparently to allow smooth aperture changes when using video, see
http://www.petapixel.com/2011/09/27/upcoming-canon-ef-lenses-may-include-aperture-rings/
http://www.petapixel.com/2011/09/27/upcoming-canon-ef-lenses-may-include-aperture-rings/