The point is occasionally made that the crop factor of the smaller format does not effectively extend the reach of the telephoto lens. It's just a crop, no more. Now currently, that's not true because some smaller format sensors, the D2X in particular, have greater pixel density than full frame 35mm sensors such as the 1Ds2.
However, if the trend towards greater pixel density continues (and it seems likely that it will) we shall soon arrive at the position where the APS-C format really is no more than a crop. The same resolution and FoV will be possible with the same lens attached to a FF camera by simply cropping the FF image in post processing, but the FF camera will retain the advantage of greater FoV with the same lens, whatever the lens, if desired.