that may or may not be the case - but its only really of interest to those of the pixel peeping persuasion, as for those who's interst is in taking great pics , rather than carrying out lab tests, the higher end Canon DSLRs are certainly capable.
I'm not knockling the nikon range either - i'm sure if you have a suite of nikon glass already the D800 is a great camera - but i'm not seeing anything thats significantly persuasive to make me want to change all my lenses in order to get a D800 rather than 5D3