Anyway to get back to JB's original question. The upgrade to the D2X was driven by two factors - the first was better AF for UW low light wildlife photography (I shoot lots of fish behaviour and this is generally in very low light at dusk). The more important reason was feeling that I was offering clients the best UW image quality I could - or at least as good as anyone else. This was less important for editorial work, but I felt was very important for the increasing amount of advertising stuff I was doing (at the time I bought the D2X I was just building relationships with agencies in NY and London - which are now going well).
I always felt that the D100 was slightly inferior to 35mm film, and certainly inferior to the top of the range Canon 1D2 Mk2. When images are selling for good money it is nice to know that your output image quality is as good as any they will get.
I am sure that you agree, John. When you were working as an advertising pro you needed 2 1/4 rather than 35mm to be taken seriously. For my market read D2X vs D100.
Anyway, all that said, I think that the D200 is probably a better UW camera than the D2X for nearly everyone.
I am very happy with my D2X, which has now done 28,000 UW images. And a few have come out OK. It may not have won me as many competitions as my D100 shots, but it has enabled me to buy a new car (well new to me, I am not doing that well)!
Alex
p.s. And finally, would I upgrade again? Yes!