Game, set, and match. That's the killing blow of Guerilla games thrashing any hope to ever see co-op come to Killzone 2. According to Co-founder and Managing Director of Guerrilla Games Hermen Hulst, Killzone 2 will never receive co-op as downloadable content.
"We looked at all the different flavors of co-op early on, but decided our first priority would have to be the single-player experience and the multiplayer," Hulst told Game Informer. "Rather than just tack on co-op, it would have to be integrated and done to a similar standard as the single and multiplayer. Having looked at it again now. I do not think we would want to add it as DLC"
Well that's odd, because when I played through Killzone 2 it seemed that there was always an AI Buddy by your side, something perfectly suitable for co-op, and definitely not something tacked on. Looks like it's back to 50 Cent: Blood on the Sand, I hear it's better anyway.