Ways a terrorist could smuggle stuff on a plane that is not addressed by any sort of security measures:

- Contraband can be smuggled onboard undetected in any number of body cavities
- Airport grounds are basically unpatrolled. It would be easy to get something on a plane without going through terminal security at all.
- There's the explosives not detected by xrays thing
- I keep wondering why someone just doesn't bring on a vial of cesium that they break and drop down the lav toilet. Seems to me that would make a pretty good bang. There could be plenty of ways to conceal it or package it so that it's not suspicious. I would imagine it could not be detected by dogs.
- Cleaning supplies still sit unattended on custodian carts past security, including chlorine and amonia-based products.
- Did I mention we all have several viable body cavities?

Ways the security policies are ludicrously inconsistent and meaningless:

- Sharp and pointy objects are banned from carryon luggage, but the first class cabin still serves food and drinks on glass plates, and glass stemware.
- Weapons can be made out of countless items permitted through security, including cords, musical instruments, tin can lids, etc
- There are other mass transportation vehicles that could cause as much or greater damage than planes that don't have TSA security. These include cruise ships, buses, trains, subways, ferries, etc


Conclusion: The current measures only inconvenience the law-abiding and only cause the bad guys to work incrementally harder and be slightly more innovative in their schemes. It does nothing to prevent someone who is motivated from carrying out an act of terroristic malice.

We could say that the situation is analogous to gun control: the only people who will abide by the regulations are the innocent. In the case of using mass transit to kill people, the only ones who will be stopped from bringing explosives, weapons, etc onto a plane are those who wouldn't do it to begin with.