As much as we want a D400... it's a narrow market... but with the type of people on forums like us, there's a lot of demand from the somewhat price sensitive DX market for an expensive DX camera... it may not come so soon after the announcement of a D4, D800, D800E, and D600. I wish it did, but perhaps I wouldn't buy one right away, maybe in 6-`12 months.
Just no reason for Nikon to rush things, especially with the D7000 price drop... and remember when the D7000 came out it significantly out sold the D300s.
Unless someone really understands the value of the differences between the D300s and the D7000, it's a tough sell. For the masses, how do we differentiate build quality? Autofocus performance? Processing speed?
My story? Here's why I bought a D300s years ago... one feature that I needed:
I fell in love with the pro-quality build of the camera, and the decent AF system. Since that I sold it and bought a D700 because I wanted better ISO and AF performance, better glass, but would buy another D300-ish camera if it had a faster AF and better ISO. I also have D5100 as a compact travel camera.
Now that I know more, I wish (like many of us) that the flagship DX camera was also aimed at the advanced photographers without any auto or scene setting clutter, without any video clutter, quick access to exposure/ISO/aperture/WB/Qual/AF controls, solid quality build and feel, large 100% viefinder, fast processor and AF, high ISO performance, and all of the attributes currently on the D300/D700. No more than 24MP and preferably less, I prefer quality to quantity, and don't want to deal with huge RAW files. That's a narrower market than Nikon might want to appease, but it's also a lot of us who spend our free time screwing around on the DSLR forums