Sounds very interesting. I would speculate that they would have produced this along with the first Horus Heresy book a while before now if it was just about getting more people into Forgeworld. I can't see them producing it now unless:
1) They wanted to see whether the Horus Heresy would be popular first.
2) This has something to do with the rumoured merger of GW, Forgeworld and Black Library all into one entity with the new website launch next year.
Personally rather skeptical about this one, although I would love to see it happen and I would be right in there as soon as it was released.