To me, the DX sensor has always been a compromise, because I shot film for so many years before going to digital in 2007 (I've been shooting since 1967). I am about to upgrade from my D300 to a D700 or D800. the D800 is my choice, but finances will determine which path I actually take. All my lenses are old film lenses that work just as well, or better, on the digital bodies than the new DX lenses; so an upgrade will be fairly painless. And I have yet to sell a camera body. I use them until they wear out, which I've never had happen. I have three film SLR's still sitting on my shelf ready to be used.
More to the point, I suspect the DX sensor's days are numbered. While it does give an extra tightening of lens scope, that's all I can say for it. The FX sensor is, next to film, the best available way to capture images: dynamic range, megapixels, focus sharpness, ISO speed and the return to standard printing sizes all argue for a wholesale move to FX. That's why the pro cameras are already there, and consumer cameras always follow the pro's technology eventually.
So I don't think there will be a D400. There likely will be upgrades of the existing DX bodies for a few years, but eventually FX will pushed down into the whole line.