Serious question for the physicists: How feasible would it be to put a faraday cage in one's underpants that wouldn't trigger a metal detector but would mess up the body scanner?
Your best bet would be to construct a garment from a material which is significantly absorbs/attenuates the extremely high frequencies (30GHz to 300GHz) used by these millimeter-wave imaging scanners. Normal clothing passes these waves with minimal attenuation, where they bounce off of your skin and return to the scanner. The scanner then uses these returned signals to reconstruct an image of your naked body for the TSA to examine. If your clothing instead absorbs most of this signal, or at least attenuates it significantly enough to fall below the dynamic range of the imager, the TSA won't be able to see much.
Absorbing millimeter-wave signals to a significant enough degree will require a relatively thick material. I typically use RF foam in the lab when I'm working on EMI compliance issues because it is cheap and easy to cut to size. Attenuation data is scarce for the EHF band, but ARC Technologies has a lossy flexible foam that has 25dB of attenuation at 40GHz ( http://www.arc-tech.com/lossyflexiblefoam.php ), which amounts to 50dB of round-trip attenuation from the imager to your skin back to the imager. 50dB loss means only 0.001% of the original signal will bounce back to the scanner.
Even more absorption can be had if you use pyramidal foam, much like you see in a typical antenna chamber. ( http://www.ets-lindgren.com/pdf/absorber.pdf ) Of course, this probably won't be the most comfortable garment you've ever worn.
Still, I would imagine wearing such a garment would immediately raise red flags and you would quickly find yourself in a back room with some unfriendly TSA agents. You're much better off opting out.
Hell, it would also make the pat-down a lot more interesting!
Thanks for the detailed answer.
Of course my original question was somewhat facetious, but I do think it's farcical that, nine years after 9/11, in order to fly within the "land of the free and home of the brave" one has to be subjected to these humiliating so-called security measures. Military-industrial complex indeed! It would be interesting to trace the political donations given by the companies that make these machines...