A very timely discussion - with BOTH consumer compacts AND dslr's pushing the megapixel bar up at every turn. With compact cameras, the high-megapixel units offer literally nothing over a 6-megapixel compact with good image quality.
The 1DsIII isn't really like that though. Canon has made drastic changes to the body (I've tried the 1DmIII) and the user interface that make it a joy to use compared to the older body. No longer do you have to press down two buttons while turning a dial to make adjustments to ISO...sigh. Now you can press and release a button, THEN turn a dial and that's it. Gone too are the days of hitting menu, select, turn, select (and hold it down!) and turn turn turn...ugh. The battery is finally as good as the D2x battery. These two changes to the camera body make it as fun and easy to use as a D2x - finally.
Also added is 14bit DIGICIII processing. I've very interested to see what a sunball shot will look like from one of these camera. IF noise performance is also improved (and I'm sure it will be) then higher ISO shooting results will be better. Paul - have you compared the 1Ds files taken at ISO400 to a 5D or 1DmIII? ISO400 is something we use underwater fairly often, and an ISO400 image from the 5D looks like ISO100 from the 1Ds. Shoot the 1Ds at ISO400 and you're now comparing to ISO1600 on the newer cameras.
I shoot ISO1600 all the time topside (make a fair amount of money w/ these types of shots) and look fwd to better ISO1600 images from the new Canon.
So - like Eric I'm not that interested in more megapixels, but other improvements. When you look at the spec, you're only actually getting 700 more pixels in the horizontal direction. Not that much really.
Sincerely,
James