Just to add, Nikon's own press release suggests that the bulk of the improvement is due to the in-camera JPG engine:
[quote author=Nikon]
Thanks in part to the new EXPEED 4 processing engine, the Nikon D4S now shatters the perception of what’s possible with available light photography, giving users a native ISO range from 100-25,600 expandable from 50 ISO (Lo-1) to a staggering 409,600 ISO (Hi-4). This amazing ISO range rewrites the rulebook for available-light shooting, and is especially useful for professional sports shooters, photojournalists, as well as military and law enforcement agencies. The powerful EXPEED 4 image-processing engine incorporates an entirely new algorithm for even higher ISO noise reduction and color fidelity, resulting in an enhanced overall sharpness and clarity without sacrificing subtle textures and luminous details.
[/quote]
Then again, given Nikon's history, they might be applying NR to the RAW files, as well. :-X