To make the tests images, I made lens-cap-on shots in manual mode, f/5.6 @ 1/250s, for each possible ISO setting. Then I processed each image in Photoshop CS5, first amplifying the noise to the max by applying auto levels, then cropping to a 3x magnification of the pixels. If there is any noise, this technique will uncover it. Here is my own video of he results:
I'm happy to say that shooting at ISO 100 on the D700 does not add noise at all. In fact, the camera is noise free from ISO 100 to ISO 800. However, shooting at any ISO other than a camera's nominal speed results in some loss of dynamic range.