I have a 30D, 40D, 50D and 1D3. On Saturday I went birding and I took the 50D and 1D3 together with my 100-400 and 1.4X teleconverter. I left home with the lens on the 1D3 but when I got to the bird reserve I saw there was nothing large enough or close enough for the 1D3 to make a worthwhile shot, and little going on in the way of action to demand the 1D3 AF performance.
Without taking one shot with the 1D3 I swapped the lens over and tried for some smaller, perched birds with the 50D. That still wasn't enough so I added the teleconverter.
Here is a sample from the day, taken with the 50D at 560mm, f/8 (wide open), 1/640, 400 ISO. This has been cropped to 1/4 of the frame and then resized to 50%. By using the 50D instead of the 1D3 I placed 2.27X as many pixels over my subject, thus picking up detail the 1D3 would have missed entirely. To get a similar sized image from the 1D3 I would have had to crop and resize to 100%, and it would still have been a bit smaller.
It's a shame it isn't a little sharper, although this has had no edits other than the crop and some output sharpening on resize and conversion to JPEG. I guess I could have tweaked my capture sharpening first. I should also have stopped down a bit. You can see how narrow the DOF is from the wire upon which the bird is perched. Then there is the wide open softness of the zoom to consider as well. I could have traded 1/3 stop or 2/3 stop of aperture for a little reduction in shutter speed. This was focused using Live View Contrast Detect AF, a feature which the 1D3 does not even have.
There is clearly room for different sensor sizes and pixel densities. Higher densities give you more detail, at the expense of noise. Lower densities give less noise but less detail as well. Larger sensors require longer glass to create an image of equal relative size on the sensor and to gain any value at all from the larger sensor. That costs big bucks for the camera and bigger bucks for the glass.
There is also extra bulk and weight to consider. When I go on vacation, with photography in mind, I like to take a couple of bodies - one to have long glass, like my 100-400. for wildlife - and one to have a wide angle for landscapes or a medium zoom for people etc.. The extra bulk and weight of the 1D3 is significant and for long range/cropping the low pixel density is an issue, compared to the 50D. For such trips the 1D3 will stay at home and I'll take my 40D and 50D.