A D400 looks more unlikely as time goes on.

I think there is room in the market for a D300S successor, but the best time to announce it, imho, was at the same time as the D600. Doing that, if they keep the same numbering system, would let people know how they sit together in the Nikon line up.
Also, announcing them at the same time would let people know from the starting RRP where the cameras stand in Nikon's pricing structure. The longer the D600 is out before a D400 appears, the closer in price they will be. Of course there is nothing to stop them pricing a D400 the same, or more than a D600, but I think they would like to keep all the model ranges differentiated by price.
By bringing out a D400, I don't think Nikon would be adding any more complexity to their range, as they will have the same number of cameras.
D3000 = D3200
D5000 = D5100
D90 = D7000 or replacement
D300 = D400
D700 = D600
D3X = D800
D3 = D4
A D300 and a D700 didn't confuse the market, so I don't think a D400 and D600 would either. :shrug: If you're spending £1500+ on a camera, you should know what you are buying and why.
It's whether Nikon and Canon think there is a market for a high performance, ruggedly built DX camera. :shrug: If they both think no, then Sony may eventually get a lot of the people who want a camera like that if they can live with their EVF, which you would imagine will only get better. If just Nikon think no, then they leave that market open for Canon with a 7DII if they get the features right.
For the enthusiast, while they may be brand loyal, but they probably value that the camera that does what they want more, and are willing to pay, to a point, not D4 prices though.
The earthquake and tsunami have obviously had an effect on Nikon's plans, but the enthusiast is more likely to spend on lenses and accessories. If they want to let that market go, they will leave a gaping hole for someone else to fill imho. :shrug: