Its helpful, first, to understand how metal detectors work. and the reasons they are used for security purposes. Basically, a metal detector emits an electro-magnetic field that flows until it comes across something metallic. The metal object disrupts this flow in different ways, depending on its size and the type of metal. The metal detector is designed to give an audible signal when the field is disturbed.
Security metal detectors are used to look for dangerous objects knives, guns, explosives, etc. Adjustments can be made to the metal detector to determine how sensitive it is based on the level of security needed. If necessary, a metal detector could be set to detect a very small amount of metal.
The problem with setting metal detectors to be over-sensitive is that it causes too many false alarms. Security areas at large public events would be completely congested with people setting off the alarms with keys, jewelry, pens, etc. It would make the job of actually finding dangerous objects nearly impossible. Therefore, metal detector sensitivity is usually not set that high.
My most recent trip through airport security (September, 2006) which was extremely tight with its new rules about liquids and how many personal items you could carry on, my 8 piercings didn't cause any issues walking through the metal detector. Neither did my underwire bra. But the penny I failed to remove from my pocket was caught by both the walk-through and the wand. The wand also reacted to the metal buttons on my jeans, but not my piercings.
In general, a few piercings usually does not pose a threat of setting off an alarm. However, with todays heightened security, it is always a possibility. The risk increases with the more piercings you have or the larger gauge jewelry you wear in them.
If you really dont want to have to explain your piercing(s), your best option would probably be to remove the jewelry before you get there.*
Have you ever set off a metal detector or know someone who did? Share your stories on the message board.
*If your piercing is new (less than 6 months) it is really not recommended that you remove your jewelry unless absolutely necessary. Even a healed piercing can close up quickly and make replacing the jewelry difficult and painful. Since your chances of setting off the metal detector are very slim, it might be best to just risk it. However, if setting off an alarm could cause serious problems for whatever reason, and removing the jewelry is your only option, please let a professional help you put them back in.