Good, thoughtful analyses on everyone's part. And the variety of answers is like the varieties of ways we all approach and use our cameras -- and it is done very civily, I might add!
When I got my first D700, the transition from the D200 to the D700 was what I termed "revolutionary" vice "evolutionary." The sudden shift from a DX sensor to a next generation FX sensor was shocking -- in a very good way. I still acquire pictures every week that take my breath away. Not that they are that terribly spectacular, but that the D700 is allowing me to do things I simply could not do before. I believe that the D700 was a breaking point in digital photography that took that art, or science, beyond what film could offer, both in terms of sharpness and low light capability. Where as I felt a lot of lenses were capable on the D200, I gradually was dragged acquire the Nikon 14-24mm, 24-70mm f2.8s and the 70-200mm f2.8 VR II. The D700 simply demanded the best in lenses to best enable it to reach its capabilities -- and of course, the best skills.
I suspect the D800 is another giant leap, albeit not the same magnitude as the D200 to D700 (or D3) was. Do I have to have one? No. Am I plannnig to replace my D700s with D800s? No. Would I like to have one? Yup. Will I? At some point, Yup. If I win the Power Ball or mega Millions will I get a whole bunch more Nikon gear? Yup.
But right now, in the real world of small newspaper photography and my other commercial work, the D700 continues to amaze and wow me. Until people stop telling me "You must have a really good lens..." I am pretty darned satisfied.