Nikonrumors see the D600 as more likely and gives more specs

My belief is that there is nothing to keep Nikon from doing this, if the distinguishing features are clear enough. And in this case, they would surely be:

1. D600: FX version of D7000 at around $1500. Enthusiast model all the way
2. D400: DX version of D800, at around $1900. "Semi-pro" "pro"

But, the D600 could be made a bit more expensive, and the D400 somewhat cheaper. The purely pro DX shooters may be a bit too few, therefore there is a price pressure for Nikon to attract more enthusiasts for the D400. Lower price will also make it more attractive as a backup/supplement for FX shooters.

Economically, the D600 may be most important for Nikon - may attract quite a few system switchers.

Strategically, the D400 is probably most important. It will come when Nikon has a good enough sensor for it, and the electronics to make it fast enough. That's not trivial. The D800 was in testing last summer. Go figure.