I was reading over at DPreview that the D610 has a new, different shutter that allows it to shoot at 6 fps and have a "quiet mode". It also has improved WB capabilities, and nothing else beyond that.
Does anyone believe there is a physically different shutter in the camera, or could it could all be the product of firmware tweaks? If we consider the latter, what would it take to hack the D610 firmware to run on the D600? Obviously it would have to be hacked to ignore the model number, but beyond that, unless there actually is a different shutter in the D610, a firmware hack could give you a D610 for the price of (an understandably risky) firmware update. If nothing else, however, the D600 could get improved WB.