Bruce Schneier takes another look at airport security after the underwear-bomber:
To the extent security failed, it failed before Abdulmutallab even got to the airport. Why was he issued an American visa? Why didn't anyone follow up on his father's tip? While I'm sure there are things to be improved and fixed, remember that everything is obvious in hindsight.
After the fact, it's easy to point to the bits of evidence and claim that someone should have "connected the dots." But before the fact, when there millions of dots -- some important but the vast majority unimportant -- uncovering plots is a lot harder.
Despite this, the proposed fixes focus on the details of the plot rather than the broad threat. We're going to install full-body scanners, even though there are lots of ways to hide PETN -- stuff it in a body cavity, spread it thin on a garment -- from the machines. We're going to profile people traveling from 14 countries, even though it's easy for a terrorist to travel from a different country. Seating requirements for the last hour of flight were the most ridiculous example.