With my lenses the D800 just isn't relevant to me - at the moment my comparison is the 500D because that's the camera I'm going to upgrade from.
I'm expecting MUCH better noise handling in the high ISO range than what I have now:
Canon EOS 500D
Canon EF 100-400mm 4.5-5.6L IS USM @400mm
ISO 3200 (and you can definitely see the noise)
But OK, let's get back to the D800 in comparison.
The 5D3 has the better autofocus
(<--a test, summary starts at 5:25), it's faster (fps and "flushing the buffer" <--- "continous shooting" - almost at the end of the page
) and for me the only thing I'm missing from the 22MP sensor of the 5D3 is the ability to crop. The images themselves are big enough at 22MP.
So while the D800 offers more detail and and gives more room for cropping I don't think that's enough to get me switching the brands, especially as I already have and like my Canon lenses. The 5D3 is more expensive at the moment but I'm not going to buy it now but some months in the future and the price will have fallen until then. And I guess I'd have to add "a bigger HDD for the D800's files" to the money you have to pay for the D800