Bruce Schneier has an excellent article on why current invasive screening procedures are ineffective at stopping bombs, guns and knives from getting on planes, and has suggestions for how to introduce effective security.
One thing, though: his article mentions that in Europe, the ineffective and time-consuming process of separately X-raying laptops isn't practiced, but on my last flights out of Heathrow and Amsterdam airports, last week, I had to take my laptop out for a separate screen.
Of course, this isn't just bad because it wastes time -- it's also a problem because it lets the whole world, including laptop thieves, eyeball every laptop entering the airport. Plus every time you have to hold your shoes, coat, belt, ticket, ID, sweater and laptop while shuffling toward the X-ray machine, there's a chance that you're going to drop your computer and smash it to flinders. We have a security procedure designed for people with nine arms.
It seems like every time someone tests airport security, airport security fails. In tests between November 2001 and February 2002, screeners missed 70 percent of knives, 30 percent of guns and 60 percent of (fake) bombs. And recently (see also this), testers were able to smuggle bomb-making parts through airport security in 21 of 21 attempts. It makes you wonder why we're all putting our laptops in a separate bin and taking off our shoes. (Although we should all be glad that Richard Reid wasn't the "underwear bomber.")
The failure to detect bomb-making parts is easier to understand. Break up something into small enough parts, and it's going to slip past the screeners pretty easily. The explosive material won't show up on the metal detector, and the associated electronics can look benign when disassembled. This isn't even a new problem. It's widely believed that the Chechen women who blew up the two Russian planes in August 2004 probably smuggled their bombs aboard the planes in pieces.