I suspect the D800 is different in this regard than previous models since it touts uncompressed live video over HDMI. I'm assuming that the D800 will be about to output at 1080p over HDMI for the live video at least. But this will be something to watch for.
If it turns out that the D800 can output 1080p-30p/24p and outputs over HDMI in the 4-2-2 color space, this will be an impressive video device for sure for film-makers who want to capture uncompressed video. For the rest of us, 1080p compressed video using the file on the card, should be pretty darn good too.