Since both the Christmas bomber incident and the Time Square bomb incident have happened in rapid succession, you may be under the impression that it is—or has become—easy to launch a terrorist attack in the United States (as it only seems to be operator error that has stopped both attacks).
1: Hard to Pull Off Terrorism sounds easy, but the actual attack is the easiest part.
2: Few Terrorists Terrorism doesn't occur without terrorists, and they are far rarer than popular opinion would have it.
3: Small Attacks Aren't Enough An act of terrorism that doesn't instill terror in the target population is a failure, even if people die. And an act of terrorism that doesn't impress the terrorists' allies is not very effective, either.
Schneier excepts "nut cases operating alone" like "the Fort Hood shooter, the guy who flew his plane into the Austin IRS office, the anthrax mailer." Those attacks are far easier to pull off than an attack from Al Qaeda. Schneier's larger points are fascinating and well worth reading, especially if you've felt a little concerned for your safety.