An official announcement hasn't yet been made, but the DLC will be coming out simultaneously for the PC and the 360. Supposed to be adding new levels and other stuff. It's the upcoming patch that isn't guaranteed to be out so quick for the 360.

This is a co-operative multiplayer game. I personally wouldn't recommend it to someone who had no way to play with other people online, especially because of the limited content. Lots of fun online, I could keep playing this for a long time even with the few levels. The special AI feature makes the game a little different every time, so while the geography of the levels is always the same, the encounters get mixed up. And the whole thing is like playing a Zombie movie, so playing with friends online with voice chat can make for some hilarious conversation.

I like how the story is presented too. The built-in story is completely optional, but sometimes the best story is the story of how you and your team actually survived (or didn't survive) the campaign.