More Nikon D600 rumors
Here are few more details about the rumored full frame Nikon D600 DSLR camera. Any information so far in future cannot be considered as reliable:
- The price of the D600 is rumored to be very low - maybe as low as $1500.
- The sensor inside the D600 will probably be 24MP (made by Sony, modified by Nikon).
- The D600 will probably not have an internal AF motor, which means it will work only with AF-S lenses (just like the D3200 and D5100).
- The D600 will have HD video.
- Announcement before Photokina (September 2012).
- One or more low-priced f/4 lenses will be announced with the D600. For example, Nikon recently filed a patent for a 24-70mm f/3.5-4.5 full frame lens which seems to be designed for a cheaper FX DSLR body.

The above image was published earlier on the Chinese photography forum xitek.com - it could be fake.
For some reason Google has captured several appearances of the phrase "Nikon D600" on the main nikon.com website:

Read more on NikonRumors.com: More Nikon D600 rumors | Nikon Rumors
Older News
On several occasions in the past, Nikon representatives mentioned that the D800 is not the replacement of the D700 but “a whole new category of camera”. There are rumors of a new, cheaper full frame DSLR camera from Nikon that will be called D600. Here are the preliminary specs:
- Unknown MP count at that point
- The new camera will be marketed as an entry level full frame camera
- Auto DX crop mode
- Dual SD card slots
- In-camera RAW editor
- Built in time-lapse functionality
- There will be no build-in HDR feature
- Possible integrated GPS
- New external battery grip
- To be released this summer
As usual, information about products that will be released several months in the future are not very reliable, so I would rate this rumors at 40% probability for now. If this turns out to be true, it will change the whole dynamic in the Nikon product line - maybe we will end up with three DX models D3x00, D5x00 and a high end D7x00 (all with 4 digits names) and the rest will be full frame cameras - D600, D800, D4 (with 1 and 3 digits names). I still believe that Nikon will release two more DSLR cameras in 2012.
References:
1. More Nikon D600 rumors | Nikon Rumors
2. Another full frame DSLR on the horizon: Nikon D600 | Nikon Rumors