>I think the D40 may have used electronic shuttering to get >1/500 sync instead of moving the shutter blades that fast. I >think the D70 did that to.
Yes, that is correct. The D600 has a purely mechanical shutter, so could not get up to 1/500th. Whether Nikon should have allowed it to go to 1/250th is a matter of opinion; personally I'd be quite happy with 1/200th instead