That's a good question. I've been looking forward to it for a while (it was announced on Steam over 2 months ago) but so far, nothing.
My guess is that they are waiting for the stand-alone expansion and then they'll release both in some sort of bundle.
Hopefully it will happen soon as Steam is the only way I can (legally) acquire the game where I live, since Direct2drive doesn't sell games to non-US residents.