I fly dozens of times a year--always wearing an underwire--and it's never set off the metal detector. I primarily wear Wacoal brand, but also Chantelle, DKNY and Agent Provocateur. If I happen to get randomly selected post-WTMD for a hand-held scan, the bra alarms, so I'm sure the underwire is metal.
For those of you who are absolutely sure it's your bra, can you say with 100% certainty that there's no other metal on your body? No wedding rings? No other jewelry? No jeans with metal buttons & rivets?