Coming soon to an airport near you: instead of a printed boarding pass, you might be able to just scan your phone. United Airlines has just started its paperless boarding pass program in eight U.S. airports with the potential to grow. Passengers can receive their boarding pass by email and simply show it via their phones to the security officials. According to the TSA blog, "The paperless boarding pass will ... prevent fraudulent paper boarding passes that could be created and printed from home."
As long as your cellphone battery doesn't die, and there isn't some other hardware or software problem with your phone, and your email to the phone is working, etc. .... then this sounds like a timesaver.