I would think Nikon would have some naming convention.
For example, the D (single digit) series like D1, D2, and D3 and their variations are built tough for extensive professional use.
On the other end of the line is the D (four digit) series like the D3000 and D3100.
In the middle used to be the D (two digit) series like the D80 and the D90.
That seemed to make sense to me, the smaller the number after the D, the more professional the camera's design and intended user.
But then complexity set in. The replacement for the D90 is not another double digit camera but rather is a quad digit camera: the D7000. Why? To avoid confusion because there was an old D100? Another complexity: a D (triple digit) camera should slot between a double digit camera like the D90 and a quad digit camera like the D5100, But it doesn't. Nikon's triple digit camera slots between the single digit D3 series cameras and the D7000 which is the new top of the line for the old D double digit line of cameras.
There used to be four lines, single digit, double digit, triple digit, and quad digit. Now I think we will have three lines: single digit (D4) signifying built for professionals, triple digit (D400 and D800) for professionals who don't want a "brick" or want a lightweight backup or for non-professionals who want that FX size sensor, and quad digit (D3100, D5100, D7000) for different levels of non-professionals. I suggest the double digit line is dead and now is incorporated into the high end of the quad digit line. But where does this leave the new D400? I suggest it will be the DX equivalent of the FX D800 serving the "semi-professional - advanced amateur" who prefers the DX senor over the FX sensor for some reason such as compatibility with an existing collection of DX lenses or use in telephoto wildlife photography.
If I am correct, I would expect to see a new D4 series over the next few years such as a D4s and D4x before we get to the D5. Also, I would expect the D800 and the D500 to essentially share the same body and both use the same battery holder. Also, they should evolve to a D800s and D500s about two years from now. Finally, I would expect the D quad series of cameras to be Nikon's best sellers because they are all the camera 90% of purchasers really need. The D7000 already has a rugged magnesium alloy body. In a few years a D7100 or D8000 will be out with an improved sensor (perhaps the 24 mega pixel sensor rumored to be in the new D400) so what more will anyone really need?
Since digital cameras advance so rapidly (every two years significant changes are made) couldn't a professional (such as a wedding photographer) be as well served by using a D7000 and replacing it as soon as a newer version is out? Will not the D7000 magnesium alloy body with its 150k shutter actuations (enough for 205 shots every day for two years) actually serve a working professional adequately at less than half the cost of shooting the D3 or D4 series?
If you see errors in my thought, please inform me.