I took some photos in a very, very dark venue at ISO 6400 the other night and they came out with some added help from noise ninja. Everyone else around me just put their cameras away. Basically, if the D3s has double the ISO capabilities of the D3, i.e. 12,800 without going into the extended mode, it should be quite a capable camera in very, very low light .
Personally, I don't care for the video mode since I have a video camera that has sat in a drawer for a long time--I just don't want to take the time to edit anything; however, the low light capability and already installed "optional" buffer is appealing.
The thing is I guess is how low of light does one want to shoot if they don't need to shoot in that dark of conditions and how fast one wants to fill up a card in RAW. Of course, someone into survellience would probably love the D3s.