OK, when the 24mp sensor in D800 body?
At least as you describe it.
with 6+ fps, as the true replacement for the D700.
You really don't get what the D700 was. The D700 was a lower priced D3; thus, the D700 replacement would be a lower priced D4. Given that, expect a D4 sensor in a D800 body, expect it to cost about the same as the D800, and expect it to have 5 fps without an optional grip and 8 fps with an optional grip.
Jan or Feb 2013?
While the D700 came out 11 months after the D3, things were a bit different then. Back then Nikon only had one FX camera and nothing to compete with the lower priced 5D; today Nikon currently has three FX cameras covering the entire range of prices offered by the competition. Nikon is not compelled to undercut their flagship model now, so it stands to reason they won't. I would be very surprised if there was a D700 equivalent camera before summer, 2013; I wouldn't be very surprised if it wasn't available until 2014, or even not at all.