Quote>> Does anyone know if Nikon are officially saying D800 is successor to D700?<<Quote<<
Not that I know of, but the D700 is being phased out, first in Japan due to their new battery laws. I think the D800 is a replacement for both the D700 and D3x.
The D800 does share much of the same layout with a very similar body. The physical size of the FullFrame Sensor is almost the same. But it seems to share more of the advanced technology with the V1, D7000, and D4.