I stumbled onto this thread and thought that as a d800 owner, I'd chime in with my strident, biassed, obnoxious thoughts
Despite not having used a 5D III, all my research suggests it's a great camera, and probably better as an all-purpose camera than the d800. I see the d800 as a camera that's capable of doing anything, but that has very specialized strengths. Those strengths happen to appeal to me: I'm a convert from large format photography (when I started doing color work I could no longer afford the film and processing, and MF digital is beyond my reach). Resolution and dynamic range are so important to me that I'll make big sacrifices for them. Shooting speed, autofocus, high ISO abilities etc. are cool, but I'm not going to make sacrifices for those. This may put me a minority among photographers, and d800 is for that minority.
Dynamic range isn't just about not knowing how to expose. If you don't control your lighting, you quickly find that the world offers way more than ten stops of range in everyday scenes. Shooting black and white 4x5 I frequently had to do N- (pull) development to get 15 or more stops at a useable range. With the d800 I get a bit over 12 without the noise becoming objectionable. This is a big deal for me in the kinds of urban landscapes and unlit interiors that I do. For the many photographers who control their lighting or shoot in lower contrast settings, this ability may be completely unimportant. For me the 2+ stops advantage over the 5D is worth more than the resolution advantage (which seems to be about 14% in real world results).
If I depended on AF, shooting speed, video, or if I did massive volumes of work and didn't want to deal with file sizes that are often gratuitously large, the 5D would be much more attractive. It also fits my hands better (based on picking it up at the store) but my camera stays on a tripod almost all the time. I suspect that if you shoot hand-held, the pixel difference will be very hard to discern.
Justsomeolddude pretty much nailed it. These are different cameras ideally suited for different markets. The annoying thing for Nikon users is that Nikon currently doesn't have a camera with the strengths of the 5D … just as Canon doesn't have one with the strengths of the d800. This forces a lot of apples/oranges comparisons, and green grass / fence discrepancies. It will be interesting to see how the one-upmanship progresses over the next few years.