While terrorists may be costing us a fair amount of of money, aggravation, and freedom, they do appear to be failing pretty badly in one area: that of blowing things up.
More than two years ago, Lewis Page, an ex-bomb-disposal-operator, wrote an article for The Register describing how incompetently made most bombs are. He's recently written a new article explaining how they really could blow up things if they got their act together, found a few competent people to build the devices, and found some others to deliver them. Given the lack of successful attacks—heck, given the lack of attacks period—it appears as if that's much harder than it looks.
Or perhaps there are some master terrorists somewhere that realize they don't really need to go that far, and that a few clownish failed attempts (which stand little chance of doing much harm beyond the suicide bomber himself) are entirely adequate to give entire nations a panic attack.