Via Andrew, a very interesting study:

Racial profiling, in other words, doesn't work because it devotes heightened resources to innocent people -- and then devotes those resources to them repeatedly even after they've been cleared as innocent the first time. The actual terrorists, meanwhile, may sneak through while Transportation Security Administration agents are focusing their limited attention on the wrong passengers.

It is important to remember that the most successful interdiction of a terrorist -- an unknowing terrorist, in this case -- trying to board an airplane with a bomb is the 1986 case of Anne-Marie Murphy, a red-headed, pregnant Irishwoman whose Palestinian boyfriend, Nezar Hindawi, planted a bomb in her carry-on bag in the hope that it would explode on an El Al flight leaving Heathrow for Israel. Murphy passed through the normal Heathrow inspection, but El Al security sensed something suspicious about her. Questioning -- not a pat-down, mind you, but a series of questions -- eventually led El Al to the bomb. If El Al security had merely been racially profiling, this red-headed woman would have boarded the flight, and 375 people would have died.