It will either be 16 MPX with the same high ISO performance as the D4
- OR -
It will be 36 mpx with high ISO performance slightly better than the D7000 - whatever the engineers could eek out in the rather short time frame between the development of those two sensors.
If the latter it should be called D400x, with a D4ish sensor on a D400. Otherwise Nikon will have killed a great concept in the D700 (In my very humble opinion).
There is no compelling marketing reason to go beyond 16 mpx now that Canon has drawn a line there. If Nikon were to go to 24-36 Mpx it would be to leap way ahead in that race. And there may be no major competitors in that race. I think that argues more for 16 mpx but I wouldn't be surprised with either.
I'm not a sensor engineer so I'm not sure how hard it is to get a usable yield with that many photo sites but that is probably the only inherent technical reason not to build a 36 mpx sensor. It would just be a D7000 sensor (more or less) scaled up in size.
It's pretty obvious something is coming, and soon, likely two bodies since there is nothing current in the market now in between the D7000 and D4. That can't stand for long (I hope).
Edit: we should all consider red, white and green ink. Don't forget the pickles or relish!