For those who ask, the Nikon D3x is not yet confirmed by Nikon. However, this is the supposed/rumoured name of the next Nikon Full Frame camera (after the Nikon D3 and D700). It is supposed to use the Sony 24MP sensor that is at the core of the next Sony Alpha 900.
Interestingly, Thom Hogan, who is often quite knowledgeable about Nikon, thinks that the name should become the Nikon D900. He thinks that this would be the reflection of an effort to bring own the price to allow it to fight against the real competition seen as the Canon EOS 5D MkII and the Sony Alpha 900. Honestly, I have no opinion or no information to add here. You know that I think this would be needed (in terms of price) and names are mostly a marketing issue after the technical features are locked in.
The availability date should not be more than 1 year from now, and quite probably in the very first weeks of 2009.