Three airlines now let passengers board with electronic boarding passes on their mobile devices—American Airlines is the latest, joining Delta and Continental.

The service is limited to certain airports and only one passenger can check-in and pass through security using the handheld, which means if you booked your reservation with anyone else you'll still have to print a boarding pass.

When you do have the option, using the mobile boarding pass requires you have a functional PDA or smartphone with an Internet connection that you use to check in and have your boarding pass sent to. At the airport, the bar code on your phone will be scanned twice, once when you check in and again as you pass through security, ostensibly offering a higher level of security than what's offered to paper-boarding-pass holders.

At last, a security improvement that has the potential to actually lessen your load. As you take off your shoes, remove your laptop from your briefcase and fold up your jacket to fit on the conveyor, you don't have to worry about what to do with that precious tiny scrap of paper that is your boarding pass.

On the downside, clamping your driver's license and smartphone between your lips isn't recommended.

American launched the mobile boarding pass option at O'Hare, Continental passengers can exercise it at Houston, New York/Newark Liberty and LaGuardia, Boston, Austin, San Antonio, Cleveland and Ronald Reagan Washington National Airport. Delta offers the service for flights leaving LaGuardia.

Typically you must be boarding a non-stop flight.