5DIII and D800 are completely different cameras. So why would you try to compare them?
Because I have both of them on pre-order, and whichever is the better camera will determine which system I go with.
Because I'm heavily invested in the Canon system, I want the 5DIII to be better, but it's hard to ignore the Nikon.
Because both the 5DIII and D800 represent the mid-range, prosumer DLSR in each company's respective lineup, both are priced similarly, and both will be compared to each other by potential consumers.
Because like it or not, they are not completely different cameras. They're competing in the same arena of the market for the same consumer dollars. The 5DII and D700 were different tools, with the former aimed at resolution and the latter at speed, but people inevitably shopped them against each other.
As long digital cameras are being sold there always has been the trade of between amount of pixels and amount of noise. Also you can not expect large files and high fps together. All this is getting better with every camera but the difference remains. Just get what you need most.
The 1Ds, 1DsII, and 1DsIII all offered an outstanding tradeoff between resolution, speed, and noise. On paper, the 5DIII continues this tradition. The problem is that the competition has been making revolutionary strides, and the D800 sacrifices very little in speed and noise compared to the 5DIII despite its massive resolution advantage. Canon used to be the class of the field in this regard, and Nikon was an absolute joke just five years ago. Unfortunately for Canon shooters like me, the tables are turning.
Before the sample images from each body started floating around, I hoped the noise and DR of the 5DIII would have been improved enough to make the D800 less appealing. It doesn't look like Canon has succeeded.
Again, this is still merely speculation and I'll reserve final judgement until I can test both cameras out in the field. That said, I'm still pulling for Canon.