From Bruce Schneier at Wired:
Regardless of the threat, from the would-be bombers' perspective, the explosives and planes were merely tactics. Their goal was to cause terror, and in that they've succeeded.
Imagine for a moment what would have happened if they had blown up 10 planes. There would be canceled flights, chaos at airports, bans on carry-on luggage, world leaders talking tough new security measures, political posturing and all sorts of false alarms as jittery people panicked. To a lesser degree, that's basically what's happening right now.
Our politicians help the terrorists every time they use fear as a campaign tactic. The press helps every time it writes scare stories about the plot and the threat. And if we're terrified, and we share that fear, we help. All of these actions intensify and repeat the terrorists' actions, and increase the effects of their terror.