Where that rumour falls down for me are the dual cf/sd slots as I see Canon going CF only with their pro lines.
As far as the rumoured price, well you would be getting getting what looks like 1DX AF with a larger FF sensor. The MkII was £2,300 when launched so $3,500 sounds about right.
You might have to wait a long time for the D700 to lower in price.
I would also pick up a D700 if they became cheaply available but I doubt that will happen and so the D800 just makes more sense for little extra money.
Keeping my eyes open but I think soon enough price difference between a new D800 and a second hand D700 will be around £400 and the D800 is definitely worth that extra.
The D800 will drop to sub £2000 before too long.
Tbh, I don't think a D700 is that much of an upgrade is it?
I'm sorry but why do a lot of people insist on high frame rates for BIFS. I can understand it should make it easier but surely learning proper technique is only a matter of time and experience. I have a d800 on order and I'm sure I will appreciate the 4 FPS it delivers.