If you read the link in my first post you would have noticed someone who has tested pre-production samples of both the D800 and the D4, he claims that both are the same for video and as the D4 appears to output 1080i, there is no reason why the D800 should be any different.
Check the comments section:
http://www.dslrnewsshooter.com/2012/01/07/nikons-james-banfield-demonstrates-the-video-functions-of-the-d4/

Again, these are pre-production samples, Nikon might change the final spec (but I personally doubt it).

Neill

Website: www.epicdream.co.uk