It does seem that the D3s has about a one stop high-ISO advantage over the D3. However, I think there's something else going on which may not be confined to high-ISO performance, and that's the higher resolution of the D3s. It's particularly noticeable in the Rob Galbrath's shots from the D3s at, for example, ISO 3200 which is sharper (or slightly more detailed) than the D3 shot at ISO 3200. In fact, the D3s shot at ISO 3200 is even slightly sharper than the D3 shot at ISO 1600, but one wonders if the slower shutter speed at ISO 1600 has a bearing on this, considering the lens used does not have VR.
I get the impression from other comments that the D3s has an improved AA filter, and maybe better micro-lenses. If this is so, then one would expect the D3s to be slightly sharper and more detailed than the D3 at all ISOs.