DPReview just posted its high ISO studio tests for the Nikon D600, both in raw and JPEG. The results are consistent with what we have seen from the Imaging Resource studio comparison samples.
The comparison tool is here:
DPReview also posted more sample photos. What's interesting is that they mentioned that they took the shots in raw and converted them using a "private beta" of ACR 7.3. Hopefully this means that raw support for the D600 is just around the corner for the rest of us.