While it's not inconceivable that the D800 could have a 36MP pixel sensor - just taking the existing D7000 sensor tech and scaling it to full frame would more or less provide that, I can't help thinking that it would be overkill. Chatting with a pro photographer the other day, we came to the conclusion that FF with 18-20MP would be the most likely resolution of the the D700's successor.
Besides, until Nikon actually announce the D800, or whatever it's going to be called, nobody, except those withing Nikon, knows what the specs are going to be, making this thread, interesting as it may be, pretty much nothing more than idle speculation.