I have gorgeously detailed 24"x36" prints made from shots taken with my D300. When viewed from normal distances for that size of print (you need to be far enough away to take the whole thing in with normal vision), sharpness, fine detail and everything else is professionally salable. 12 megapixels is more than enough for all practical intents and purposes when doing output at 300dpi.
If someone is shooting large landscapes or fashion for reproduction at line screens of 2400 and higher, medium format is the only way to go.
So my view and experience is that at 12 megapixels, full frame, Nikon has found the ideal blend of resolution for print coupled with excellent low noise/high ISO performance. I pulled a shot off my D3 (looking past the spire of Westminster Abbey toward the HP/Big Ben) taken at 9:45 PM on June 30. It's a nice shot of both the spire and Big Ben mainly, framed by trees, shot while standing on the corner of Great Smith & Victoria Street, with a cloudless pale sky in the upper background. Shot with the D300 at ISO1600, it's a decent shot, but printing at large sizes is going to reveal the noise and Noise Ninja hurts the focus. But the same shot with the D3 at 3200 is just superb. There's no reason to think that the D700 won't do just as well.
My point is that once you get past about 10 megapixels, it's no longer megapixels that count most of the time. Noise, color accuracy, contrast control and dynamic range become far more important factors.
More megapixels don't fix or ameliorate noise problems. More megapixels don't improve dynamic range. And since 12 megapixels already provides an incredible amount of detail, critical shooting at very large viewing or output sizes should probably best be done in medium format.
I read somewhere (Rockwell's blog maybe?) that between 20-24 megapixels (depending on the quality of the lens being used), most currently available 35mm glass is no longer effectively usable because these powerful sensors begin recording actual flaws in the glass itself.