It has been a big concern for me also, as the option for me otherwise would be perhaps to either wait to see what Canon comes up with, or look at the D800.
So, I've been following the subject closely for the past year or so. Of course I've seen the rumors about production for the D400 starting to ramp up in Sendai about four months ago, the several "reliable" sources with bits of information (who knows what that's worth) and of course all of the behaviors and analyses aimed at predicting what Nikon will do, and how their strategy is segmenting the market with products.
To me, the most defining and recent confirmation was the Photokina interview with Nikon Europe's Product Manager for Professional Imaging. The subject was the D600, and one of the questions/answers were as follows:
Dirk - is the D600 the new D300S? In other words a bridge between your DX format DSLRs and high-end full-frame?
"It's hard to compare the D600 to the D300S. I see this camera sitting between the D7000 and D800, which are closer to the D600 than the D300S is. It has a decent resolution - 24MP full-frame - and we've included some of the features of the D7000, in terms of accessibility, and the scene modes and so on, but also we have D800 technology like the HDMI output, uncompressed video recording, the metering capabilities, which are adapted from D800 and D4-level technology. The D300S is a different concept. It's a semi-pro DX system camera and the D600 is an enthusiast camera. So really we're talking about two different types of product."
That statement alone could be analyzed (maybe over-analyzed) thoroughly, and perhaps should be, as this person is intimately involved with new product strategy. What he didn't say is also very important. He didn't indicate that the D600 is the direction Nikon sees for those looking to replace the D300S, although he would concede that those D300S users who hesitated to upgrade to the D700 due to the cost, would now have a more compelling reason to. The other thing he didn't say was that the D400 is coming, and of course he couldn't release that information.
Even more obvious than that, if we look at the big D400 gap in the Nikon lineup, and the D7000, D600, D800 have been positioned in a way to clearly make room for the D400 market segment and the associated future sales. From the perspective of pricing, build quality, AF system specifications, bracketing capability, there's an intentional gap.
What can perhaps be confusing is the lack of pro-build DX "only" lenses. That isn't worrisome to me, the pro DX market is too narrow to demand it's own lineup of lenses when several levels of DX and FX lenses are available and compatible.