I'll believe it when I see it. I still believe that Nikon has decided that "pros", and amateurs with pro money, should upgrade to FX,...and buy expensive FX glass if they want to shoot wildlife or sports with a pro body. All of Nikon's recent moves point in this direction. After all, when was the last "pro" caliber DX lens upgrade?
For the rest of us, we get the D7100 for DX, and the D600 for full-frame. The D600 is a hell of a camera that is 95% of what a D400 would be, for probably less than half what a D400 would cost. We should probably count our blessings.
Even if it was possible to put the D7100 sensor in a D400 body and give it 8-9 FPS and a massive/super fast buffer, and then layer on a superior AF system to the current Cam 3500 system, it would have to cost $2,500 or more. At that price, for a DX camera, I just don't think there would be enough takers (and this site would disproportionately contribute to that number) to justify the investment/development/marketing cost.
If I'm not mistaken, when the D200 came out, there was no full-frame. When the D300 came out, there was no D700, and the full-frame D3 cost four times as much. Now, you can get full-frame in a compact body for under $2,000, and full-frame will serve you better than DX for everything but wildlife, sports, and other extreme tele needs. IMO, that limits a D400's appeal to a very small market. I'm not saying those in that market don't have a need, but just that there aren't as many of us as we think there are,...and probably not enough to matter to Nikon.