Stan made some good points earlier about the amount of development required to pull off a D400 camera. The D4 aside, Nikon has not produced a camera over 12 MP that can hit 8 frames a second yet. To get there with a 24 MP sensor is going to cost money, and it's not just a bigger buffer, pretty much everything has to be updated. That requires a serious commitment to the pro DX format, that I am not sure Nikon wants to make.
Therefore I am not convinced that a D400 is a done deal. I have read a lot recently that indicates different Nikon executives themselves are not on the same page regarding a DX in general and the D400 specifically. Have you taken a look at the new 70D is has a separate AF button, 7 fps with out a grip and a buffer twice the size of the one in the D7100. It has a whole new video focus system that no other DLSR has. We will not know for a couple of months how good the sensor is, but I bet it has been updated.
I think Canon can leverage enough from the 70D and the 5D MII to bring the cost down on a new 7D MII. They don't mind selling cameras with 20 MP sensors, so that also brings cost down while delivering good frame rates. True Canon did not have the best sensors, but with the 5D MII and the 70D I think that is changing. As much as I would like to see a D400 come out, I don't think just because Canon is likely to come out with a 7D MII, that Nikon will follow and deliver a D400. Nikon will have to justify their development cost and want to invest in the high end DX market. I bet Nikon would rather spend their yen on a high end mirror less camera that has more Of a DX future.