"Is it Nikon's desire to force them into the FX line with its more expensive bodies (D800 min if you want the D300 form factor) and more expensive options for long lenses?"
Well, yeah. I've been saying this for a while: Nikon expects pros to shoot FX. If you want the pro control layout, it appears that you are going to have to shoot FX.
The new D7100, with it's professional AF module and D4 Algorithms, is now superior to the D300s in every conceivable way except frame rate. Control layout is a personal preference, since the D7100 has 99% of the button controls of the D300s (except for the lonely AF-On button). They are just in a different place on the body.
Unless there is a new, superior professional AF system, that Nikon hasn't put on the D4 or the D800, waiting in the wings for a D400, I can't imagine what practical upgrades Nikon could put on a D400 DX body that the D7100 doesn't already have. Does it make sense for Nikon to simply put D7100 innards into a D300 body, just so some users can avoid adapting to a new interface, and so some users can have a bigger, manhood-compensating, "pro" body?
I could be wrong, but with the D7100 release, I really don't see Nikon releasing a D400 for the exclusive practical use of working pros switching between FX and DX bodies on a daily basis, or to satisfy the practical needs of a few guys with big hands.