Damn, don't you love all of the arguing? I called a reputable camera store and had a salesman tell how much better the 5d3 was regarding IQ versus my 5d2. His exact words: "The sensor is a completely new architecture and produces MUCH BETTER RESOLUTION and COLORS!!"
I find that extremely hard to believe. He did state that the ISO 12,800 shots were astonishing, which I have to generally agree with. They are nice, from what I've seen online. I will not upgrade to the 5d3 unless I "master" my mk2 and it cannot do something I need it to.
Regarding the D800, yes the DR looks unreal. That is Nikon's (or Sony's I should say) bread and butter. They thrive in that technical area. To me, I always thought Canons produced nicer colors. I did preorder a D800 and can't wait to do some landscapes. But if I was going for one system vs. the other, Canon has a much better selection of affordable glass. And their telephotos are much newer. Short primes, OTOH, are what Nikon has been devoting all R&D towards. Each has their strength. choose what you like and tread lightly on others.