GamersHell spotted that the Ubisoft's UK listing for Soviet Assault has the game dated for March 13th.
The expansion to World in Conflict will be released in boxed form as the World in Conflict Complete Edition, which will include the original game in addition to Soviet Assault, although those who own the original will be able to download the expansion online.
Soviet Assault switches sides, with players following the events of the original game from the perspective of the Soviet invaders. You can expect a new campaign, four new characters, extra multiplayer maps, and likely more.
World in Conflict was released in September 2007 to high acclaim, so this expansion's been a long time coming. March 13th, comrades.