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My D800 has image area selection for FX, 1.2x, 5:4, and DX crops.
Why would anyone want a DX only professional camera?
Makes no sense, so I feel pretty sure Nikon will never makes another one. After all, wasn't the D600 moniker an intrusion into the DX body naming space? I think it was.
It might be worth your while to read Thom Hogan's thoughts on the subject; here's three articles to get you started
But from my perspective:
Well, for starters, the D400 is expected to be 16+ MP and have 8 FPS. Those extra 3 FPS are significant for actioin photography.
Then there is the cost factor: the D800 is $3000 where as a D400 should be around $1700 to $1800. That's a saving of 40+%. I don't know about you, but I don't have $1200 to waste on FX when DX suits my needs better.
Then there is the size and weight factor. It's reasonable to assume the D400 will be about the same size and weight as a D300s. The D800 weighs in at 2.2lbs vs. the D300s at 1.65lbs. A savings of 25%. In addition, the D300s is 14% smaller by volume.
Now do the same exercise for the glass that matches the FOV on the two formats. For instace, on a D300 (D400) a 300 f/2.8 must be replaced by a 400. In that ONE lens there is thousands of $$$$ of savings and a significant size and weight reduction. Even at the mid-range, the 17-55 is smaller, lighter, less expensive and covers a greater range of FOVs than the 24-70.
DX poses a value proposition for many people that is the perfect ballance of performance, price and bulk.