] Profiling has two very dangerous failure modes. The first
] one is obvious. Profiling's intent is to divide people
] into two categories: people who may be evildoers and need
] to be screened more carefully, and people who are less
] likely to be evildoers and can be screened less
] carefully.
]
] But any such system will create a third, and very
] dangerous, category: evildoers who don't fit the profile.
] Oklahoma City bomber Timothy McVeigh, Washington-area
] sniper John Allen Muhammed and many of the Sept. 11
] terrorists had no previous links to terrorism. The
] Unabomber taught mathematics at UC Berkeley. The
] Palestinians have demonstrated that they can recruit
] suicide bombers with no previous record of anti-Israeli
] activities. Even the Sept. 11 hijackers went out of their
] way to establish a normal-looking profile; frequent-flier
] numbers, a history of first-class travel and so on.
] Evildoers can also engage in identity theft, and steal
] the identity -- and profile -- of an honest person.
] Profiling can result in less security by giving certain
] people an easy way to skirt security.

Bruce Schneier, trying to spread the clue around.

How We Are Fighting the War on Terrorism / IDs and the illusion of security