"The traditional alerts on radio and TV are still important, and they will continue, but more and more, mobile devices are becoming essential. You have them with you."
"In the event of a major disaster, government authorities can get lifesaving information to you quickly."
These alerts will be send via messages by the carrier (which will be notified by emergency officials) you are subscribed to. As of now, all four major US carriers have agreed to participate in this program, which is dubbed the Personal Localized Alerting Network (PLAN). According to FCC's plan, this service should be available throughout the nation by April 2012, but Verizon, AT&T, Sprint and T-Mobile have all pledged to support this initiative before the said deadline. It should go live in New York and Washington DC by the end of this year.
These alerts will be geographically targeted, i.e. if you are in California, you won't receive an alert about a disaster happening in another part of the country. You will be able to unsubscribe from all of them (issued by local, state or government agencies) except for presidential alerts.
It's important to note that not all phones have the chipset that's required to get this service, and, moreover, even if they have it (like some iPhones and Android devices), these handsets will still need to receive a software update to enable it. It's said that AT&T phones scheduled to be released in October 2011 will have FCC's PLAN (with all alerts) pre-installed. What's more, at some point in the future carriers will be obligated to inform customers whether a certain phone supports this service or not.
source: USA Today via Electronista