They developers have repeatedly stated that they didn't think the Tyranids could be done justice with the current DoW engine. Wether that's true or not is debatable, but I take it as a pretty clear sign that they're planning on incorporating them into DoW2 somehow.
Anyways, I'm pretty meh about this annoucement. Everyone has known this game was on the way for a while, but nobody has had any details. Now they tell us the game is in production, and that's it. I'll start drooling when the first screenshots and such arrive.