Due to the difference in controls, I don't see the D600 as a replacement for D700 any more than D7000 was a replacement for D300s, even if the newer models offer superior IQ to their bigger brothers.
Nikon has done a funny thing in offering a camera in the D800 that was such a radical jump forward in pixel count, it left a lot of D700 users wanting a logical D700 replacement, which would be a D4 sensor/ISO performance, moderately high frame rate, in the semi-pro body. It's possible Nikon looked at the D700 and thought it cannibalized D3/D3s sales too much and decided to not do that again. Anyway, at this point I don't see any doubt that D800 was the D700 replacement, just not the camera a lot of D700 enthusiasts were looking for.
The D600 will further push down used D700 prices. That's just the way the game's played. I'd buy a few if we saw a blowout the likes of which happened with the D200 a while back, but it looks like there probably aren't enough D700s out there to make that happen.