It's not co-op, but the game it's based on is. Amazon is now listing the Halo Trilogy soundtrack as shipping December 2nd. The box set will contain all 5 discs from Halo, Halo 2 and Halo 3. It'll set you back just shy of $30, but it's a small price to pay for one of the best original scores in video game music.
Source: Amazon.com