> But that's the point that Nate and I and others are trying to make:  the "no 
> liquids" rule constitutes spending millions on security that does just about 
> nothing to prevent attacks.

These rules do little to improve safety if you assume that terrorist 
organizations are run by scientific masterminds who are capable of 
designing and implementing arbitrarily sophisticated attacks. Empirically 
speaking, this does not appear to be true. Most of the things that 
terrorists do are quite crude, e.g., strapping bombs to themselves, 
sneaking guns or knives onto planes, driving explosive-laden vehicles 
into buildings, leaving explosives in trash cans, etc. Straightforward 
countermeasures include chemical detection of explosives, behavioral 
profiling, X-rays, pat downs, putting concrete barriers around important 
buildings, and not placing trash cans in public places (as is the policy 
in the London Underground). To say that these measures do "just about 
nothing" for safety ascribes too much cleverness to the standard 
terrorist and ignores the common modes for terrorist attacks; if such an 
attitude were implemented as public policy, it would likely lead to an 
increase in terrorist attacks. It is correct that we cannot protect 
against arbitrarily Byzantine terrorist attacks, but such things are rare. 
If you look at Wikipedia's (by no means comprehensive) list of famous 
terrorist attacks:
   http://en.wikipedia.org/wiki/Death_toll#Terrorism
. . . it seems that the vast majority of attacks used the very simple 
mechanisms enumerated above. If we have such empirical data about how 
terrorist attacks are typically implemented, perhaps amplified by specific 
intelligence about impending attacks, we should definitely use this 
information to check for the most obvious things.

I agree that the ban on liquids should not consist of security officers 
asking you to be earnest in answering the question "do you have any 
liquids?" However, reasonable implementations of policies like this will 
improve security because many terrorist attacks are simple and can be 
stopped by simple countermeasures. This is particularly true if the target 
is a place like an airport where entry and exit is tightly controlled. 
This is less true in places like a hotel or an open air market.

~j