I cant see sport photographers using the D800 at 4fps
Definitely not a sports camera. Neither is the Mk III, really, though it would clearly be a better choice than the D800.
A few things I've noticed from various sources:
1. The D800's buffer/card-writing speed is something that seems to bother a lot of users.
2. The D800's AF may not be up to snuff for the 36mp sensor. The entire sensor, which is apparently the same unit used in the D700, is entirely f/5.6 and f/8 precision, as opposed to the Mk III's sensor which has a mix of f/4 and f/2.8 precision sensors, along with f/5.6 precision sensors. This can make a huge difference with large aperture lenses.
3. There's a bit of complaining about the grip.
4. There's quite a bit of complaining about the screen having bad color.
5. The comparison reviews I've seen so far from independent sites seem to be leaning (so far) toward the 5D Mk III as a more versatile, usable, camera.
I don't state these as a put down of the D800; it's clearly a formidable camera with the highest image quality of any digital camera yet produced. It's just that for all of the complaining I've seen about the Mk III (mostly asking why it's not a D800), it looks like Canon actually did a very good job making a well-rounded camera. In day-to-day use, I think it's really hit the sweet spot.