While we've only seen one piece of Mass Effect DLC for the 360, Bioware assures us that new planets, races and missions are still on the way in future expansions. So most of us would expect PC players to one day get all the same stuff. But according to Bioware's Matt Atwood, such is not necessarily the case.
We're asked if we're going to do Bring Down the Sky for PC, and it's a consideration. We're looking in to exactly what we want to do; maybe we'll do something different, because the PC offers some different potentials.
Is that a hint at a complete expansion? We'd usually think so, if Bioware weren't trying to crank out three Mass Effects in a single product cycle.
Mass Effect Interview [Eurogamer]