A PC Gamer preview explains that the Tyranid campaign is a bit more RTS-y than others, letting you build a number of units in the field by capturing outposts. There'll also be unlockable units and upgrades. PC Gamer further reveals that are "at least" four campaigns in Retribution, each more than 15 missions long.
Retribution is slated for release in March 2011, using Valve's Steamworks suite rather than the Games for Windows platform seen in the original Dawn of War 2 and its expansion Chaos Rising. A multiplayer beta is due to run for DoW 2 owners next year.
Sweeet, <3 Tyranids