I had a bad expierence at an airport about 2 years ago. (before 9-11) I was listening to a CD in my laptop as I was walking through the airport, and I went to go through the metal detectors. I don't think the laptops have to be off when you put them on the belt, but for some reason the monitor for the machine died when the laptop went inside.

Maybe it was because of the laptop being on, or the motor spinning the CD, or just the music coming out of the headphones, but it shutdown the machine for a good 5 minutes before they got it back up, and I got a harsh verbal treatment. It was cool though. They understood, and I went to go snowboarding like I was supposed to without a hitch after than.