Bryan takes me to task for my prediction that free market airline security would still be rigorous and intrusive.  His prediction: 

..here’s one massive cost-cutting, convenience-raising change I’d predict in a free market: Profiling.  Private security firms would still claim to treat everyone equally.  But they’d wave the elderly, women, families with children, and well-dressed men right on through, especially if they look like “regular Americans.”  

Of course, actual terrorists have already figured out the well-dressed thing; here’s one person going through security on a certain September morning we all recall; these two people boarded AA 77 at an airport a few miles from my house the same day.  All three quite well dressed. 
Here’s another hypothesis put forward by Bryan:
Private security firms would redirect their spare attention to (a) young men who look like they might be violent criminals, and (b) anyone who looks like they might be of Middle Eastern descent.

Let’s focus on case (b).  As a matter of crude efficiency–leaving aside the salient issue of fairness as far outside my scope of expertise–would rigorous profiling actually work?

It depends partly on how readily al Qaeda and similar organizations can find substitutes for men who fit the profile.  You might think that we can look around at recent attempted terrorist attacks–the Shoe Bomber and the Underwear Bomber–and conclude that Bryan’s forecast is already a decade out of date. 
But things aren’t that bad for Bryan’s hypothesis: They’re worse!  And basic price theory helps explain why. 
Let’s consider two chalkboard regimes: Under one, the government treats every passenger identically, and under the other, the government focuses all attention on passengers fitting an ethnic profile.  Clearly they’re both extreme cases, and actual U.S. experience and potential profiling regimes are both somewhere along that continuum.  
If a government credibly commits to treating everyone identically, what incentive does a terrorist organization have to search for recruits who differ from the stereotypical terrorist profile?  None whatsoever.  All passengers will be treated identically, so if it’s even slightly easier to find recruits who are part of the profile (a big if), then 100% of actual airline hijackers and bombers will fit that profile.  Lowest cost sourcing.  
However, if the government (or a private security firm) switched to 100% profiling, then it’s obvious what the terrorist organization will do: Find someone outside the profile.  Past experience suggests al Qaeda has already done this, and just last week a bit more evidence came in supporting the hypothesis that ethnic profiling at the airport won’t be quite as effective as some people think, even under our current rules.  And if ethnic profiling became legal, terrorist organizations would have an even stronger incentive to recruit from non-profiled demographic groups.   
Hijacker ethnicity is an endogenous variable.  
Coda: Info on female suicide bombers here.