The DLC has been released yesterday.
I bought the Game + Underworld DLC and I've got the Heart of Gold for free.

There are 3 discount packages at the moment.

Game = 19,59 Eur (-30%)
Game + Underworld DLC 25,89 Eur (-30%)
Game + Underworld + Heart of Gold DLC 32,75 Eur (-22%)

If there is anyone in here who's going to buy this Dungeon Keeper successor in the future, buy the Game + Underworld DLC and you'll get Heart of Gold DLC for free (unless they get this fixed).