Schneier asserts that "It's hard to sneak terrorists into the U.S.", which is hard to swallow given the essentially unchecked flow of immigrants coming across the Mexican border every day. Why would a terrorist fly into a major US airport, when he could just as easily fly into Mexico and then walk across the border (carrying as much cargo as he liked). Think of the vast drug shipments that cross this border, and then tell me that sneaking across a small suitcase of biological agents would be difficult.
I also disagree that "small attacks aren't enough". If you and 9 of your friends coordinate to roll grenades into 10 elementary schools in 10 different cities, I can assure you that you'd make headlines, and bring daily life to a standstill as parents refused to send their children to school.
It's also increasingly bizarre to me that terrorists would target airports or airplanes at all. Airports are the one place where security is most concentrated. Practically speaking, blowing up a plane is probably the most difficult option you could choose if you wanted to kill 300 people. Why not pick one of the other myriad unguarded 'soft' targets where people gather?
Either terrorists are incredibly stupid, vanishingly small in number, or both.