walnist: How is the D800 better than this?The 5DmkIII has better high ISO performance, and faster shooting rate.What the D800 has going for it is the higher pixel count, but it has to be seen whether this translate in any higher effective resolution as measured in lp/mm.With such tiny photo sites only the sharpest primes will likely be able to achieve decent pixel level sharpness in the center of the image (see Canon's 7D which has the same pixel density, for comparison), and the borders will be soft no matter what.The D800 can be a good cheap studio camera (but for true 36 Mpixel you'll have to go medium format anyway), but the 5DmkIII seems more versatile and better all-rounder.
I doubt anyone who buys d800 will attach cheap zoom to it. Take a look at the first D800E sample from nikon website , does not look very soft in the corners.. It is also not true that 7D has the same pixel density - 22.3 X 14.9mm sensor hosting 18 mega pixels, sensor is 332.27mm squared, pixel Pitch is 4.3 microns, D800 - 24 X 35.9mm sensor hosting 36.3 mega pixels, sensor is 861.6mm squared, pixel Pitch is 4.88 microns.Everyone chooses products for their needs and likings, there is no same better and worse for all people. Enjoy 5dmkIII!