Doing some other research, I came across an interesting Nikon patent, filed on February 3, 2004. The patent describes a fisheye lens that can be used with both full frame, smaller digital sensors and APS format cameras:
«The invention provides a fisheye lens system having an angle of view of 170 degrees or more capable of being used with a plurality of cameras having different image formats in size.»
A multi purpose fisheye (or other wide angle) lens would be very useful for photographers carrying different format DSLRs in the future.
http://www.nikonweb.com/patent6987623/
Jarle