If it's a disposable plastic lighter, just keep it in your pocket - it won't set off the metal detector. If you send it through the bag screener, airport personnel will make you throw it out.

When coming back from Hawaii last week I had some time to kill at the Maui airport before embarking on our gruelling 9-hour flight(s) back to Houston (2 layovers - thanks United!). I was in the smoking area inside the airport and saw a grumpy looking pilot pull out some matches to light up. Having already thrown away my disposable lighter at the airport entrance, I asked the guy for a light. Here's how the conversation went:

me: Thanks for the light. I just threw away a brand new lighter.
pilot: What kind was it?
me: Just a cheap plastic disposable.
pilot: You can take it with you on the plane. Just keep it in your pocket.

me: Really? It doesn't set off the metal detector?
pilot: Nope. But if you put it on the belt they'll make you throw it out.

You learn something new everyday. Sarah thought it was a little disheartening that the pilot volunteered this information so quickly. But she doesn't understand the relationship that fellow smokers have.

UPDATE: As of August 2007, lighters are now allowed on planes.