<<The big question is why is Nikon taking so long with this body? Are they not committed to it or is a big change coming in one of the key structures (AF perhaps?)>>
It's all hypothesis at present of course, however like Nikon, Canon have been doing similar things. Nikon with it's D7100, Canon with its 70D! I think a bit of brinkmanship is going on here. I have no doubt whatsoever that a D400 will arrive, I also think it's price may make potential customers take a very deep intake of breath. Also, are they preparing the market for a camera with a run of at least 4-5 years as with the D300s.
Who knows, I am getting a distinct impression of a few folks thinking, "who cares"! I am amazed what I've managed to accomplish with my D7100 and I think Nikon know they've produced a great model there. My D7100 will give me quite a few years, by then if it does arrive who knows, the D400 may even be affordable.
So, hypothesis maybe, but when the D400 is announced, light the touch paper and watch the forum here ignite In short I know nothing, which is a bit like many of the Nikon commentators I think.