B&H Photo is listing the D700 as discontinued! Click here to see.
By all accounts the D600 is going to be a full-frame 24MP camera with the D7000 layout. If this is true, the D600 is not a replacement to the D700, but a mid-level model with scene modes, a lesser build quality, and a smaller size with a smaller battery. In effect, not a camera that a pro would necessarily want.
As a pro, I need a full magnesium weather-sealed body, no extraneous scene modes, more dedicated buttons, and stronger batteries.
This puts the Nikon line in a strange spot. The D800 isn't a replacement for the D700, too many megapixels and the high ISO isn't as impressive as Nikon and other websites lead you to believe. I shoot both side by side in low light situations and the D700 is superior hands down. The DX line is still topped out by the D300s which is surpassed by the D7000 in specs, but not in build quality and pro features.
An FX D600 would likely kill the Nikon DX pro-level cameras. If you can get a full-frame camera for around $1500 why pay $1800 for a DX camera? Then again, there's the build quality issue, and some sports and nature photographers prefer the extra reach that a 1.5X sensor provides, so maybe a D400 DX camera is in the future.
So what of a relatively low resolution D700 replacement? Possibly a 16MP sensor in a D800 body? I'd trade my D800 for a 16MP camera in a second. I don't need the 36MP for most of my work, but I DO need two camera bodies that are similar in form and function as I use two bodies simultaneously a lot of the time. Though the D700 and D800 and minor differences, they are similarly laid out and I can switch between the two effortlessly. Backing up a D700 or a D800 with a D7000 isn't a feasible option for most pros, the completely different layout causes you to have to pause between camera changes to get oriented with the button layout, this means missed shots.
I guess we'll just have to wait it out to see, and with PMA on the horizon I don't think the wait will be long. I'm betting on at least two cameras to be announced.