Because it's 2012, and every action game ever needs co-op, Spec-Ops: The Line will receive downloadable cooperative missions. No word if the entire campaign will be co-opified (a word that is totally not made up), but the download will be free. So that's nice, at least.
But all of this begs the question: why not release it on the disc? Sure, the game is already gold since the release date is so close. But why not develop it alongside the game if you know you're going to release it? This industry sometimes.