I'm curious about the S5 as well. I might rent one to check out its files, especially regarding the dynamic range.
I shoot mostly cityscapes and some architecture, some people - all with the D200. I like the speed of the D200 for when doing portraits, as well as wireless flash. But if significant enough, I would trade better files for slower camera.
Shooting landscape you could say that it would be really nice to have the extra dynamic range. Same thing applies to resolution though. Both lack of dynamic range as well as resolution you can deal with by shooting multiple shots. But it seems to me I would much rather have the resolution, and bracket for extended dynamic range, than have the dynamic range but having to stitch a panorama together. Both is possible, but the first is much easier (and if absolutely necessary sometimes even doable by hand with the D200's 5fps, well ok perhaps not really).
To me it would all depend on how much nicer the colours of the S5 are, or how much less resolution/detail there is.
With the D200 I shoot mostly RAW and if I don't bracket, I develop two or three versions of the same file to get that little bit of extra detail and colour in the shadows, and a smoother transition to the highlights.
If a single exposed S5 gives a similar file, I would seriously consider trading my D200 in.
Regards...