Playing around with the D800 high ISO images that have been put out there I think that it is performing better at high ISO's than my D700 is, especially when you take a D800 full FX shot and down-sample it to 12MP.
I think what some people are doing is zooming in to 100% on these ISO 6400+ shots and directly comparing them to a 100% zoom of D700 ISO 6400+ shots. That's not a valid comparison because if you print both to a 16x20, or down-sample the D800 to the same MP, the noise pattern from D800 will be much tighter due to the higher initial starting resolution.
I am getting a D800 but keeping my D700 for now. Primarily because I use the D700 with an MB-D10 for the higher 8FPS frame rate in certain situations. That extra 2-3FPS can make a difference. I don't consider the D800 a full replacement of the D700. If they had bumped it up 8FPS with the MB-D12 in the DX crop mode then maybe.