Call of Duty: Modern Warfare 3 will not include campaign co-op, Infinity Ward creative strategist Robert Bowling has confirmed via Twitter. A report yesterday suggested campaign co-op was to be included, but that has now been debunked.
"That was in reference to Special Ops coming back in #MW3 with new modes and features," tweeted Bowling. So there you go. If you want to play cooperatively in Modern Warfare 3 you'll have to make do with Special Ops and team multiplayer modes.