One of my complaints about DXO has been that they measure a sensor and declare a camera to be the best without ever taking a photo.
They have started testing lenses a while back, and publishing results in conjunction with DPR. I suspect that this has raised questions as to what the rating of a camera would be if a lens were installed.
We have long noted that a image from 6 or 10 mp sensor looks very sharp when viewed at 100%. This is due to a lens resolution, circle of confusion, and other factors. When you get high pixel densities, resolution of the system does not scale, but is always a improvement.
That is what DXO mark is saying.
Technically, the MTF of the system is equal to the MTF of the individual components multiplied together, and is always less than the weakest link. Even film has MTF values specified.
So, for simplicity, if a lens has a MTF of 0.9, a Body 0.8, together they are 0.72. Increase the body to 0.85, and the system becomes 0.765. Better, but not a revolution because the lens needs to get better as well.
However, there are those who only look at one part of the picture, the number of MP, for example and happily believe that with twice the MP, they get twice the resolution. (No reflection on CR members who mostly know better).
One thing that the D800 sensor brings is very noticeable improvement in dynamic range under harsh lighting conditions such as bright sun and deep shadows.