In his monthly Crypto-Gram newsletter, Bruce Schneier argues that people who try to bring 3.1 ounce tubes of toothpaste on a plane should either be treated the same as someone who tries to smuggle a gun or a bomb onto the plane, or else they should be allowed to bring the toothpaste (or shampoo) on the plane.
No terrorist is going to base his plot on getting a gun through airport security if there's a decent chance of getting caught, because the consequences of getting caught are too great.
Contrast that with a terrorist plot that requires a 12-ounce bottle of liquid. There's no evidence that the London liquid bombers actually had a workable plot, but assume for the moment they did. If some copycat terrorists try to bring their liquid bomb through airport security and the screeners catch them -- like they caught me with my bottle of pasta sauce -- the terrorists can simply try again. They can try again and again. They can keep trying until they succeed. Because there are no consequences to trying and failing, the screeners have to be 100 percent effective. Even if they slip up one in a hundred times, the plot can succeed.