The game may have been used to get the terrorists in question into the correct mindset, or given them ideas (ie, to attack an airport). However so many forms of media can do that. Can anyone who has played Flight Simulator honestly say that they have never gotten bored and crashed a plane into "insert famous landmark here" just for the hell of it. I'm pretty sure the designers didn't have that in mind when they made the game. As for all the action/war films out there, surely they must give terrorists ideas.
By blaming a game/movie/book/song we are ultimately losing focus on the true culprit, which is the person who actually made the conscious decision to commit the crime. The media would be better blaming whatever organisation actually committed the attack. I'm going to take an absolute stab in the dark and guess that it WASN'T Infinity Ward or Activision.