Yes I will upgrade my D700 to a D800 if it is no more than $3K and if it is no more than 24mgpx and has at least as good ISO performance. I do not want a camera with what strikes me as useless megapixels (although I will listen to arguments) that just will consume space and slow down my computer. I see no value in it.
I also agree with comments regarding "form factor", size and weight, of the probable D4. I am not a professional and the current D700 plus a good lens is weight enough. Not to mention the likely costs of a D4.
For those of us who just enjoy photography,rather than those who most compete against other photographers for business, there is a cost/benefit curve that is maxing out. In other words, the costs of a D700 produce x benefits. If I spent two or three times that on a D4, how much additional benefit would I realize in my photography? I imagine not very much. If you are a professional and you may acquire business or not lose business by saying "I have the best camera" then the cost/benefit curve looks very different.