Nikon Rumors have published specs on a supposed upcoming Nikon Full Frame camera, the D600. Reading the specs, it looks to be a D7000 body with a 24MP Full frame sensor from a D3X and a D800/D4 screen. Sounds like a winning combination to me, as I like the size and weight of the D7000 and the screen and sensor will also be a good improvement over the D700 too.
My main concern is low light performance. From reading the specs, it probably won't offer any improvement in this area over the D700, and in fact the D3X wasn't as good. Perhaps newer software and some sensor tweaks might improve things.
The other thought is the size. While I like small cameras, full frame lenses tend to be larger than DX lenses, and this camera could feel very front heavy with a Nikkor 24-70mm f2.8. Even with a lighter Nikkor 24-85mm f2.8-4 may feel unbalanced.
All in all, it sounds like the replacement camera I have been waiting for.
Here's a shot from a full frame analogue Nikon FE2.