I summarize the Nikon rumour for D700 replacement:
D800 will have a sensor carriage that can be removed by an authorized dealer. The rest of the specs: 16.4MP FX and 12MP B&W sensors, 3.3″ LCD screen with a built in collapsible hood, new MB-D10i battery grip, 1/16000sec, full HD video at 120fps.
Shutter speeds of 1/16000sec and a new technology for very long exposures without noise or extra time. (This will be very good!)
Better AF in video mode with proper Full HD video at 120fps
D3 will be replaced by D4 at the end of the year.
Read more on NikonRumors.com: http://nikonrumors.com/