They have been using the same engine, that IW engine 4.0 just tweaking it so who knows. However Robert Bowling did confirm this in a tweet.
"Robert Bowling has recently taken to Twitter to ensure the fans of the game that the developer has “moved beyond the MW2 engine”. Although Bowling, didn’t have any official details to spill on the enhancements to the engine, he did note that they have added a lot of “cool stuff” for Modern Warfare 3."
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I forgot to add my thought on COD using a new engine, I think there 1 year turn around time is what is preventing them from using a new engine. They could not create a flawless game in one year working on an entirely new engine shhhh they can't get them right now...lol COUGH COUGH UNREAL ENGINE 3............