This is certainly not meant to start a flame war (please, please do not flame me) but your situation seems ideal for the Nikon, either the D2x or the upcoming D200.
I abstained from similar thoughts, and will attempt an ecumenical response. I agree that for those of us whose resources limit our maximum focal length to, for example, 400mm, the greatest telephoto reach is achieved by having the highest sensor resolution in lines per mm, which roughly means closest pixel spacing. Because when your subject is so small or far away that you have to crop, smaller photo-sites give you the more pixels on the subject.
Currently, the D2x is the DSLR leader with 5.5 microns (ignoring the E-300/E-500, for which no 400mm lens is available) with the D200 about to be a close second at 6 microns. However the 20D and 350D not far behind at 6.4 microns, and it seems quite likely that Canon's successor to the 20D will close the sensor resolution gap on the D200, or possibly surpass it. Any of these Canon or Nikon models is a better choice for this particular situation than the Pentax and Konica-Minolta models or the 5D and 1DMkII, whose 8 micron or larger photo-sites are a potential advantage for dynamic range and low noise at high ISO, but a disadvantage for telephoto/cropping reach.