Emergency messages from the President, Amber Alerts and other messages will start coming to people by way of text messages. The service will be like what you hear on the radio - a funny bleep and a message (though, we usually hear "this is only a test). You can get them maybe as soon as April of next year if you live in New York or Washington, if you have a phone that has a special chip in it (most newer smartphones like the latest iPhone have them), and if your wireless carrier is part of the program.
From the New York Times:
Julius Genachowski, the chairman of the Federal Communications Commission, said the system was designed to minimize the ability of hackers or spammers to send fraudulent messages. The alerts could appear directly on a cellphone screen, for example, rather than appear as a text message notification, and would probably be accompanied by a special vibration or other kind of signal.
"We don't expect the alerts to be frequent," Mr. Genachowski said. "They will be reserved for when they are truly needed, for tornadoes or for disasters like 9/11."