If you have flown lately, you possibly passed through one of the new "naked x-ray machines".
This measure is in response to the Christmas Day plot of 2009, when an al-qaeda operative tried to ignite a bomb concealed in his underwear.
In a recent article in the Journal of Transportation Security, two physics professors at the University of California, Leon Kaufman and Joseph W. Carlson, contend that the "naked e-ray machines", designed to find things like "underwear bombs", can easily be fooled.
The paper, entitled "An evaluation of airport x-ray backscatter units based on image characteristics," cites a wide variety of data and asserts:
"It is very likely that a large (15-20 cm in diameter), irregularly-shaped, cm-thick pancake with beveled edges, taped to the abdomen, would be invisible to this technology ironically because of its large volume, since it is easily confused with normal anatomy." Pancakes!
Equally disconcerting, the paper also states, "It is also easy to see that an object such as a wire or a box-cutter blade, taped to the side of the body, or even a small gun in the same location, will be invisible."
What's worse, as noted by the scientists, is that the problem cannot simply be solved by raising the level of X-ray exposure: "Even if exposure were to be increased significantly, normal anatomy would make a dangerous amount of plastic explosive with tapered edges difficult if not impossible to detect."
This may very well be the reason why thorough pat-downs are also being implemented: to overcome the flaws of the body scanners.
You might want to check out Steven Colbert's take on this from Comedy Central.
In full disclosure, I am the Founding Editor-in-Chief of the Journal of Transportation Security.