Check out Carnival Booth: An Algorithm for Defeating the Computer-Assisted Passenger Screening System, via Bruce Schneier's post on profiling.

Fascinating. The government's CAPS screening program actually ends up helping determined terrorists. And if we've learned anything it's that the terrorists are determined and clever.

If security matters to you, always, in all cases, enlist a real expert to help you design the system. What you may think is extra security may in fact be extra risk and exposure.

And, btw, if you don't read Bruce's blog, you really should. And you if haven't read his book, Applied Cryptography, you really really should. I mean that, go read it now, even if you don't think about security too much. It'll help you design better algorithms, understand statistics better, and just generally make you a better software designer.