P. S. To Ray: the distinction of EF-S and DX as "cropping" rather than just being a different, smaller format depends on how well suited the lens system available for them is. With a sufficient range of EF-S/DX lenses, along with longer focal length lenses that work quite well with both those formats and 35mm, "cropping" just become another word for "also compatible with some lenses designed for another, larger format". Pentax 35mm film cameras can also use Pentax MF lenses, but no-one calls them "cropping" on that basis. On the other hand, all currently available DMF systems are to varying degrees "cropping" as their lens systems have at best been minimally adapted to the FOV needs of sensor formats smaller than the MF film formats.
There is also a distinction to be made between a format that can incidentally accept lenses designed for a larger format, and a format that, at least initially, totally relies upon lenses designed for the larger format. It took a while before Canon brought out their first EF-S lens and it wasn't a particularly good lens.
We should not forget that the difference in sensor area between the Canon cropped format and 35mm is almost as great as the difference in area between 35mm film and the smallest MF film, 6x4.5 (comparing actual exposed film area).
I don't think that 35mm film for still photography would have been so successful if the users had to rely upon lenses designed for 6x4.5.
If Canon and Nikon were to design top quality lenses for their cropped formats, which were actually sharper (on average) than the full frame equivalent focal lengths, then the 'cropped format' system would become almost as expensive as a full frame system, for the serious photographer. It's the lenses which are the major cost. As I recall, my 17-55/2.8 zoom cost as much as the 40D body.
In fact, for the serious photographer, it's the availability of good lenses which is a major drawcard when choosing a system, at least for me.