I think a camera like this from Canon is inevitable, sooner or later.
The way it sits at the moment, the D800 has lots of pixels, arguably more than you need, and only runs at 4fps because the processing engine can't go any faster. The 5D3 has a lot less pixels, but it still has 'enough' and can run at 6fps as a result.
However, a key feature of the D800 is that you can switch to DX mode, still have 16mp (same as D7000) and get 6fps, plus the benefit of the extra crop factor reach. The two benefits (usually) go hand in glove for action photography.
We don't seem to hear much about this aspect of the D800, possibly because you need the battery grip to get the extra pace, but it's a killer feature IMHO. Makes the D800 two cameras in one.