I read about this a few months ago, was pretty disappointed, was hoping they'd 'surprise' us but that seems unlikely. I remember halo 3's co op being announced fairly late in development. Before the guy never really said no, he just said that they 'never said they were doing co op.' and kind of danced around the question. But now it seems he's saying no directly to the question, it's probably too late now, or whatever they were working on didn't pan out.
The co op campaign in resistance 2 is pretty cool, but would have really liked splitscreen in the single player campaign like the first game, It felt like the game was missing something cause they left it out. I'm assuming they just didn't have enough time to do it and figured the co op mode would cover it.
Now I don't think anyone complained last year when Call of Duty 4 came out with no co op just like the way killzone 2 is, yet that game went on to be huge. You still can do 'online coop' in a way by playing on the same team online. Theres alot of similarities with that game, because let's face it CoD4 did very well mostly because of it's graphics. It makes me wonder if they used the game as a model late in development.