Nikon Rumors has made a statement regarding a major new camera announcement which is to take place before PMA in Las Vegas. The date given was January 6, 2012 and the indication is that the camera may be the Nikon D4. While more is apparently known about the D800, it looks like we may see the D4 announced first. There have been few leaks and rumors about the D4, but it will be aimed at sports, and low-light photographers who need high frame rate and high-ISO to capture their images. Expect a 16MP camera with expanded ISO range upwards of 200,000 and frame rate of 11 fps. The professional flagship DSLR is expected to ship by the end of January, 2012. (Source: Nikon Rumors)
Perhaps Nikon is waiting for Canon to announce the 5D Mk III before they bring the D800. Canon has already announced the 1D X which has specs very close to the rumored D4. The 1D X is expected to be released March 2012.