when in fit to page mode scrolling down will change
pages, out side of this you can scroll down to the
bottom of the page but you can't switch pages. also
when in a double page it will scroll to the bottom and
stop, giving no real indication of the rest of the page.
not to endorse ripping off things whole sale but
http://www.geocities.com/davidayton/CDisplay uses the
following behavior that well probably should be ripped
when you scroll to the bottom of a single page it stops
scrolling but after a half turn or so of the wheel it
flips to the next page.
when the image is to large to fit in a single screen
the scroll wheel starts scrolling the image
horizontally once you reach the bottom, then in the
lower right corner of the image it acts as above.