Upload & Sell: On
I think there WILL be a D400. The D800's 6 FPS (after you add $1000 in accessories) is still a tad slow for a high-performance sports or action camera. I could see a D400 hitting 8 FPS without needing a grip - fast enough to be substantially faster than the D800, but slow enough to not encroach on D4 territory. I could also see Nikon pulling the old "a grip magically makes your camera faster" trick and limiting the D400 to 7 FPS but giving it 9FPS with grip, and then pricing the grip at almost D800 levels (which is to say, very expensive).
A D400 sits in a nice spot for the advanced amateur / cropper pro / FX backup body for a lot of people, and as said previously the $1995 price (which it will certainly be) is in a different tier than a $2995 camera, or $4000 camera if you count the costs getting the D800 to 6 FPS..
D400 might even come in at around $1695, further differentiating the tiers, who knows.