It looks like Games Workshop has set up a place holder on the GW Webstore for the Horus Heresy. The address matches up with the heading for a game, and while the page was blank, its been since removed. While it could be something for the Black Library, it's quite possibly what we have been waiting for in regards to the recent rumors of a new stand alone game.
Here is the address where it was found. Since most people missed it, the image at the top of the article is what was at the location before it was removed.