The Canon EOS-1D X is the replacement for both the EOS-1D Mark IV and EOS-1Ds Mark III, and is slated for late March 2012, just in time for the 2012 London Olympics. The “X” apparently stands for “Crossover” or “Xtreme” or even the 10th generation EOS (or something like that).
DC Watch has an interview of the execs responsible for the design and development of the camera. It is in Japanese and we provide a [rather crude] English translation via Google translate.