You may have already seen the D800 vs. D700 high ISO comparison post (and anther one) on Nikon Rumors website a while ago. The 100% crop images from Nikon D800 clearly win with less noise and more details when they are scaled down to D700 size. If you were still not convinced, here is another comparison.
A Romanian wedding photographer took a spin with the Nikon D800 at a local store and came away with raw NEF images from the camera. He also shot the same scene with a Nikon D700. Even though the images were all shot handheld and the white balances also seemed different the comparison should still give you a good idea about how the two cameras compare in the high ISO shooting situations.
From the posted images, the Nikon D800 images downsized to 12 megapixel clearly show better image quality with more details, less/finer noises at high ISO settings. The true 100% crops from Nikon D800 also look very good. Noises at pixel level are about the same but the Nikon D800 images have more details than the Nikon D700 images.
All images shown on the website were said to be converted to JPEG in Lightroom 4 without noise reduction. I found that to be incorrect. There was likely no luminance noise reduction applied but the default color noise reduction setting 25 was most likely left on in Lightroom 4. With color noise reduction turned off, the images would show blotchy color noise at high ISOs (to be expected).
Keywords: D700, D800, ISO, Nikon, Noise