If they're smart - and they are - then just run the drill two or three times until folks get used to it and slack off.Once found, we run around in circles like a dog chasing its tail as we try to figure out how to stop a ficticious attack that won't be coming, but we don't know that...yet.
Find the holes in the system like the last poster said. Plan accordingly and implement.
Americans have very short memories. The above would work a couple of times until the denser among us learned their lesson. Then the terrorists would have to come up with a new plan - NOT terribly difficult to do IMO.
What really surprises me is that they still haven't attacked the US directly since 9/11. Probably too busy in Iraq?