In the past week I received so much conflicting information about different camera models possibilities (D700s/D700x/D800/D900) that at point I cannot say for sure what will be the actual number. And then today I found out why - the rumor is that this time Nikon has sent blank bodies out for testing, without model number and nobody from the testers really knows what the final version will be. I am not sure if they have done this in the past (Leica labeled the M9 as M8 when they sent it out for testing).
Whatever the model number, the consensus seems to be higher resolution and full HD video. I have not received ISO sensitivity, viewfinder coverage or any other details yet.
Jeff-c and Thom Hogan mentioned that Nikon rotates their model numbering scheme and we are currently in the "odd cycle" D700, D90, D3x, D5000, D3000, D3s, D300s. The next one is the "even cycle" D4, D400, D600, D800. If this is true, D900 could be the real name, especially if it has higher MP that the D700.
Thom Hogans predicts release in the first two weeks of February. PMA starts on February 21st, 2010. Michel Lammerse posted online his own predictions of what will be coming from Nikon.