Unless Nikon release something special in the next 12 months, I think the natural upgrade path for a lot of D700 owners is to the second hand d3s market. I just can't see wedding and event photographers coughing up for a pair of D4's etc. (always need a backup).
I think the D700 may have been a one of a kind camera. A classic. One of the first full frame cameras that was relatively cheap, equally good in low light and sports as well as landscape photography. And the last camera to miss the whole hd video thing. I just did (still does) everything equally well.
If I ever get rid mine, I will certainly miss it.