You knew it, we knew it, but God it's nice to really know it. Dawn of War II will definitely, definitely feature Tyranids, as evidenced by Relic's making an already splendid CGI trailer even more splendid:
Drooling chitinous spacebeasts have never been so hot. Edit - in-game footage here.
IGN UK has an interview up with DOWII's senior designer Philippe Boulle about the nature of the 'nids, plus an ace, sadly watermarked screenshot of a big ol' Carnifex. Not much info, but this is the key bit:
The Tyranid swarm is a single organism, not an army of individuals. Defensive "meat shields" of disposable creatures protect powerful specialists. These specialists also serve as conduits for the guiding Hive Mind, empowering the smaller units and making them more effective. Players will actually see the swarm change tactics and behavior as it comes under the guidance of these synapse creatures.
There's also confirmation that the Space Marines will be the only playable race in the singleplayer campaign - Orks, Eldar and Tyranids are multiplayer (and presumably skirmish) only. Boo!
Thanks to Subedii and John Hilton for the tip-offs.