How does the Nikon D800/D800E perform at high iso with video?
Because the image on the sensor is downsampled* to video format, the noise is -theoretically speaking- somewhat compensated for. Since a picture says more than a thousand words I've done some testing using ISO3200 and ISO6400 at a music festival last weekend. The ISO6400 samples started to show noise and the contrast was off, but the ISO3200 samples looked a lot better. Have a look:
Technical details: Shot with the Nikon D800, 70-200VRII, 1/30s, f/8, ISO3200, incandescent, manual prefocus, tripod, 1920x1080@30fps, high quality. Audio is from the built in microphone at manual 7. Audio and video are straight from the camera and untouched except for the addition of titles at start and end.
To watch it in all of its glory on Youtube, move you mouse pointer to the bottom right corner of the video, then select '1080p HD Quality' and 'Full screen'. The movie will start to play in normal quality and switch to high quality when it its loaded. If you experience hick ups in playback, please note that they indicate a connection or server problem and are not present in the original video.
The slight flicker in the colored background lights is my fault. The power line frequency here is 50Hz, so I should have used 25fps and 1/25s, not 30fps and 1/30s (read here why).
* the Nikon D800 and D800E actually don't downsample the entire sensor image (read more here).