Terrorism on the margin

Bruce Schneier has a good post on why stepped up security at airports -- particularly in light of the thwarted London terror bombing -- is ineffective:

It's easy to defend against what the terrorists planned last time, but it's shortsighted. If we spend billions fielding liquid-analysis machines in airports and the terrorists use solid explosives, we've wasted our money. If they target shopping malls, we've wasted our money. Focusing on tactics simply forces the terrorists to make a minor modification in their plans. There are too many targets -- stadiums, schools, theaters, churches, the long line of densely packed people before airport security -- and too many ways to kill people.

His point is that a would-be terrorist has such a plethora of targets to pick from, that hardening any particular one (airports, planes) has little impact overall because there are so many substitutes. On the margin, there will always be easy alternatives to target.