It's happening, somehow. EA DICE has confirmed to PC Gamer that Battlefield: Bad Company 2's Onslaught mode is coming to PC.
The co-op focused joint, which is already being sold as DLC to console owners of the game, features four-player play similar to that of the game's existing "Rush" mode. In a nutshell, Onslaught is about you and three other dudes taking over control points in four specially retrofitted maps.
The reason DICE didn't immediately commit to taking Onslaught onto PC is because of the co-op. The studio just didn't want to sacrifice its big 32-man servers for this much tinier and more focused mode. In a bit of irony, it was the one-upsmanship with Modern Warfare 2 that put DICE in this position to begin with, since peer-to-peer networking would have removed the wait for Onslaught entirely.
PC Gamer doesn't have any information on how DICE is handling its server woes, but it's probably safe to assume that the studio has figured out a way to get around them considering Onslaught is finally coming out and all. Speaking of that, no word on price or release date. We've contacted EA to say "hey, what's up" and ask about these details.