One of the biggest issues I have with high MP count cameras is the problem with lenses which are far from perfect. While centre resolution might be quite capable of high performance, corners and borders certainly aren't, and the higher the MP count the worse the effect appears. It's not such an issue with longer lenses or primes with fast apertures which blur this area deliberately. Wide angle is the worst with lenses like the 17 - 40mm L having almost no resolution in the corners.
For me low light performance isn't such an issue, and when it is a bit dark there's always the option of flash, it's very rare for me not to have the option. If you do need low light performace then perhaps the 5D MKII/I is not the right camera for you?
Low light performance really means good high ISO performance. The 5DII is Canon's best high ISO, and I don't think Canon will let Nikon continue to walk away with the high ISO award.
High ISO is important for many things: low light use, ability to shoot at small apertures in moderate light, when very fast shutter speed is required like sports in indoor arenas. I can't think of why anyone would object to good high ISO performance. There is a trade off between pixel count and high ISO, and I hope the 5DIII achieves a good balance. Personally I don't need more pixels, so I favor improvements in ISO performance. Of course in a perfect world I'd get both in one Camera.
I think you've missunderstood my point here. By high Iso performance I'm talking about being able to shoot in a dark room where you can barely see at Iso 25600 and still get useable images (Something D3 users claim to be able to do).
High Iso is relevant because it's directly linked to dynamic range, a camera has to have good performance, but in a camera like the 5D MkII/I I don't require class leading performance, I don't want to photograph the black man looking in the dark room for the black cat that isn't there!
BTW I would have said that the ID MkIII was the best camera in the line up for low noise.
As to Nikons answer, well it doens't bother me too much as I'm not a Nikon user, but to me the 12MP sensor looked underpixeled when it was launched, and the high Iso ability might be a plus for it, but other than that it's a bit of a one trick pony, and the half stop between the D700 and the 5D MkII isn't enough for me to lose much sleep over.