I'm new to these forums, but, I'm not new to 40k and the world of GW. Matter of fact, I worked for the GW HQ US for 3 years, so, I've gotten to know the world of GW behind the scenes and know a good deal about numbers and the such.
As little as has been printed about Dawn of War, it sounds like it could be a good RTS. However, there was one point that I read that stood out that could hold this game back from being a contender with Warcraft and C&C. That is the part that Relic is only limiting the single player game to just the Space Marines. In my opinion, this is a big mistake. While the Space Marines are probably the biggest selling army for 40K, only limiting this game to just that army, is only going to make this game not so great. Sure, it was mentioned that in -multiplayer- games, you can play any of the armies that are included in the game. But, not every person out there is someone that enjoys playing computer games against other real opponents. They enjoy booting up the game and playing against the computer for an hour or two after they get home from work, or before they head off to bed. I can tell you for a fact, that at the GW HQ US, that for those people that are employed by GW there and that play 40k, that maybe a quarter of those are diehard Space Marine players. Another quarter have Space Marines as one of many armies they have bought and built. The rest play Chaos, Eldar, Dark Eldar, Ork, Necrons, Tau, and so forth. Even one player, for you old time 40k players, wanted to bring back the Squats army.
So, a word of advice from someone that has worked in the 40k industry, if you want your game to be good, just limit single player campaign to just the Space Marines. But, if you want your game to be great, you will have a campaign for each army you include in the game(Space Marine, Chaos, Ork, etc). That is why games like Warcraft, Starcraft, and C&C have lasted for years and years and years, and that is why, at least in my opinion, Homeworld wasn't as fun in my opinion, because it was one sided.
Oh, one last thing. To make this an even better game, for the multiplayer games, either allow people to make up armies(as in mods to get Space Wolves, Nurgle, Speed Freaks, etc), or, include them or have them as a download for those that enjoy one special Chapter of their favorite armies. This would separate this game from any other RTS game out there, because it can be very customizable to multiplayer gamers.