I had the chance to play with the Nikon D600 for few hours. Here are some high ISO comparisons between the D600 and the D800E:
Obviously at 916px wide, there is no visible difference between the D800E and D600 samples taken at ISO 25,600. Even at 100% crops it is still hard to see any differences:
Nikon D600 at ISO 25,600 100% crop:
Nikon D800E at ISO 25,600 downsampled to 24MP 100% crop:
Nikon D800E at ISO 25,600 original 100% crop:
Few more full size high ISO Nikon D600 vs. D800 comparisons are available on flickr (at ISO 6400, 12800, 25600):
Check also the new Nikon D600 instant rebates for October.