Yes, I am seriously considering the Canon 5D Mark III over the Nikon D800. (Have been a Nikon user since 1973 and love my Nikons, but the D800's performance, from the images I have seen, don't seem to impress me). I find the D800 noisy at anything after ISO 800. I have had a chance to play around with the Mark III and was really impressed. Also, the 4fps for the Nikon is something I don't like (the same way I feel about my D7000 when it comes to frame rate). I find the statement that down-sizing will reduce noise on the D800 quite ridiculous.
Of course these are my feeling at this juncture and can only make up my mind after I get to handle the D800.