The advertised ISO on the D700 is 200-6400. The advertised ISO on the D800 is 100 to 6400.
None of us have done a comparative test of the two in low light conditions, certainly the first thing I will test when I get mine. So, I don't see how anyone can logically conclude that the low light performance of the D800 will be any worse than the D700, which we all know is excellent.
Before it was announced I had said (perhaps to myself) that if the ISO performance remained the same, and it had the improved pixel count, the improved focusing logarithms, and the video, I was in. I am in. Maybe the actual performance will turn out to be slightly worse than the D700, but to date, there is zero evidence of that.