That's where you're wrong - the raw digital information is proportionate to the number of photons captured i.e. the area of the light sensitive area of the sensor times the exposure time. Nothing else.Even though there is more data from the 36MP sensor. Both makers use 14bit technology so there is going to be a 71% increase in raw digital information from the D800.
For a perfect computer generated image (one with no lens softening) most people would find it very hard indeed to tell the difference between a 21 MP image interpolated to 36 MP and a native 36 MP image - however thethe image was displayed. You'd almost certainly need a computer to analyse the image to be able to deduce that interpolation had been applied to one and not the other. Add on the aberrations from the lens, light scatter from imperfect coatings and diffraction softening and it's not going to make any difference at all.I know you are a fan of the 5DMKII, so am I in terms of performance. But the D800 at 36MP is going to make some portrait/wedding photographers think. At SWPP convention you see alot of A2 prints of wedding shots. Some customers want a one or two images at that size for show in their new home.
If the 5DMKIII does not have more pixels (rumours of only 22MP) they could lose the wedding market. Alot of photographers lease the kit. So a changeover is not a problem. Yes, there was the 24MP D3x but 21MP to 24MP is not alot more edge. But 21MP to 36MP that's a different issue.
In any case, with the profit margins in professional photography being what they are, I don't think many will jump ship from one brand to another even if the difference was noticeable - there's just too much investment in lenses, flash units etc. which would also need to be changed. Most pros will also be very wary of jumping from one manufacturer's kit to another because of having to re-learn handling. Of course those joining the market will be more likely to invest in the latest kit, that's always been the case.
OK, for those who think that super high pixel counts are necessary, guess the size and number of pixels in the sensor used to make this image. Or this one.