I don't think al-Qaeda is dumb enough to do anything of this sort. The reason for this is simple: Americans in general are such nihilistic, pragmatic people that their ability to accept a "disproportionate response" to this kind of situation is utterly frightening. There is no doubt in my mind that the typical American voter would be quite comfortable with the notion of killing several million largely innocent civilians in another part of the world in response to a single terrorist attack at a shopping mall in the United States.
This, in fact, is what has led me to believe that America's response to 9/11 would have been far more severe if Al Gore or (especially) Bill Clinton had been president -- because they would have had no qualms about laying waste to Kabul or Kandahar in response (just to make a political statement), and the U.S. media would have given them far more leeway than they would ever give a Republican president.
"He don't know me very well do he?" Bugs Bunny (famous American)