My friend Cullen told me the following story about Canadian airport
security:
Cullen and his family are flying to the US from Calgary. At the baggage check--before they've gone through the carry-on X-ray machine--they're selected for "random" baggage screening. The security types give their luggage a hand search and then hand it back to them. Cullen takes the bags about 15 feet away (still in plain view of security) and transfers a whole bunch of stuff from his carry-on (which, remember, has never been searched) into his checked luggage and puts the checked luggage on the belt.
Outstanding!
So, the terrorists have to rely on the effectiveness of the prescreening process to smuggle explosives into their luggage - if they don't get selected for extra searching, they'll have to explain why there's a bomb in their carry-on.
I sure hope whatever version CAPPS is at now remains ineffective enough not to aid the terrorists a lot :)
If you don't get selected for pre-screening you just throw away the contraband.