Betrayal at Calth is apparently the title of the new stand alone game we have been hearing about for months. We had earlier it was called the Battle for Ultramar, but indeed the game seems to have two forces, the Word Bearers and the Ultramarines. This is on my list to pick up when it gets released.
Remember that while still rumors, this is coming from a solid credible source. This is said to be coming in October.
via anonymous sources on Faeit 212
Horus Heresy board game is called Betrayal at Calth.
Game board seems hex-based and the dice have custom symbols on them (still D6).
Here are some previous bits of information
October Release - Very Solid