This one is coming from nikonistas.com (Google translation) – this is something like a Q&A session with Carlos Ormazabal, Nikon Product Manager for Spain, Andorra and Portugal (updated translation):
“User: Good afternoon, you know what I’m going to ask, but I have to ask it anyway. Should I buy a D700 or is the X, S, or D800 coming? Thank you very much.
Carlos: Of course I’m going to Answer.
You only have to look at Nikon’s timing… it is infallible. 2 years ago, they announced the D300 and D3… when the entire world expected the D3x. Later the D3x.
So, what would the logical continuation be after the D300s and the D3s came?
Well then. When? Count the months. How much later was the D700 after the D3? About 8 or 9 months after the announcement. So if you follow, you won’t have to wait until after summer 2010 approximately.
Now, this is only conjecture… with fundamentals, as Arguiñano (This, I believe, is a reference to a famous Spanish chef who has a TV show, like Bobby Flay, or someone like that. He recommends using the basic fundamentals when cooking. I saw him in something once a few years back. Anyway, I continue) directs, but still conjecture.
On the other hand, we can’t forget that the D800 has been announced in rumors for many months. It is not likely that Nikon will commit the same error as Cannon and produce a camera which ill be eaten alive by the MkIII’s 22MPs.
So, if you’re willing to wait for the baby D3S, count 6 to 8 months.”
If I am understanding this correctly: 6-8 months before the D700 replacement is introduced.