I may be dreaming or recovering from my NAS but you know what? I have a surprising speculation regarding the D400 specifications...
Taking into account the long dev time and the new D800 sensor..
1) A body similar to the D800 / D300 .. no surprise there ..:-)
2) a 25MP sensor
and drum roll ..
3) a 1.2 crop sensor !!
1) You will be using the sweet spot for all the FX lenses
2) You will be able to use your DX lens in 1.5 crop mode
3) You can use many of your better DX lenses in full 1.2 crop.
4) price similar to D600 .. could even be cheaper!
5) best of both worlds ..
Why did it take so long ?? I think that is the clue..
1.2 crop mode requires new sensor, new body, shutters, mirrors and prism to be developed.
Other random thoughts..
1) D7xxx remains the most advanced DX. (Clue here is that the D7000 is way more advanced than the D90 class of cameras it replaces)
2) a very attractive option for the very advanced DX users who don't want to go FX.
3) Makes it even easier for these D400 users to get fx glass (good for nikon!). since many of these advanced DX users already use FX glass on DX bodies..
4) The D800 1.2 crop mode is a big clue. a curious capability .. when I first saw it. but it could mean that the software to handle 1.2 crop is already developed.
5) A lot of the old FX glass now becomes very attractive. due to the sweet spot effect of the 1.2 crop.
6) the larger sensor will improve the perceived sharpess of DX lenses that can cover 1.2 crop coverage.
7) 25MP 1.2 crop sensor has the same pixel size as the D800.