TSAMGR, yourself, and others have commented that adjusting the sensitivity is not something simple to do and not something that is done often. I tend to believe that is correct.

My personal hypothesis on metal-detectors since summer 2001, not based on any research or even any FT searches to back it up but just personal experience at several airports:

a) Summer 2001: WTMD set insensitive enough to walk through with several keys/coins or even small pocket knife with no alarm.

b) Immediately post-9/11/2001: WTMD jacked up to very sesnsitive setting. Alarms on small belt buckles, any keys, any coins, some underwire bras, some pants, too many credit cards in wallet, etc. Still possible to go through with shoes though. And I never took off my glasses or plastic-band watch.

c) Post shoe-bomber-idiot: WTMD adjusted to cover area closer to the floor and alarm on metal in shoes. All else same as before.

--Sometime during the b-c interval, I went through the WTMD carrying no metal other than my titanium glasses, plastic-band watch, and a single dime that was left in my pocket by accident. I alarmed, apparently because of the dime. The dime was the only thing that set off the wand.

d) Later (2002-2003?): WTMDs were replaced by new units. New units were set at installation with lower sensitivity. (c) and (d) may have been the same event--I don't remember.

--I didn't pay enough attention to recognize the new units, but sometime after the second chance rule went into play I began experimenting and found I could wear my belt and wallet through without alarm.

e) 2/15/2005-- post cigarette-lighter rule--I'll be interested to see what happens here. Last week I went through with my usual metal (above) plus a bic lighter--no alarm. So if TSA is serious about enforcing this rule (personally I think they should give it lip service and no more), then they will either have to jack up the WTMD or full-body pat down everyone.