The D800 took me by surprise.
The D700 was a fantastic camera. Great bang for the buck. You essentially got 95% of the D3, probably more than that actually, at a much cheaper price.
I think what a lot of folks wanted was a higher MP camera with the same/similar/slightly better high ISO performance of the D3s/D4.
The jump from 12MP to 36MP essentially changed the product line.
Are they going to put out another camera to fill the slot that the D700 held relative to the D4?
Of course there are a bunch of people happy to see a 36MP FF camera that's $3k. No doubt about it, but I think there were a lot more people looking for a D700 upgrade with a reasonable bump in MP, high ISO performance equivalent of the D3s, and some other logical improvements such as Video, AF, etc.
So in some ironic way, the 5D3, if the rumored specs are right, would definitely fill that slot.
The D800 specs would please those who bought the 5D2, 1Ds3 or D3x, or are thinking about digital MF.
To give it higher ISO capability would require nothing more than a firmware patch. Bin two pixels together and you'll effectively get a 1-stop improvement in ISO and noise - you'd be able to shoot an 18MP file at ISO 12800 and have the same per-pixel noise as a 36MP file at ISO 6400. Overall noise at the same print size would be the same, and would be the same as any other full-frame sensor built using the same technology, regardless of resolution. The addition of sRAW or mRAW would solve the issue of file size, though.
The D800 is supposed to have high ISO capability within half a stop of the D4. The D4's better high-ISO capability has nothing to do with megapixels, though - it's due to new microlens technology. Put these microlenses on a 36MP sensor and you'd still have the same total noise as the 16MP D4. However, it may not be presently viable to fabricate the new microlenses on such a tiny scale, though, hence the D4's lower resolution.