Given the surfeit of wild and life-threatening weather events across the USA over the past few months, it's great that the major iPhone carriers now all support the Federal system for wireless emergency alerts. These location-specific messages are broadcast by cell towers in an affected area, and provide brief messages that may help provide warning or guidance in a crisis.
Verizon and Sprint both launched their capability in 2012, T-Mobile has the alerts enabled, and AT&T joined the parade last month approximately one year after the service was originally turned on. Note that emergency alert support is limited to the iPhone 4S and 5 models; neither Verizon or AT&T supports it on older handsets. (Thanks to John F. Braun for the reminder on the model restrictions.)
As long as your handset is new enough and is running iOS 6.1 or later, the alerts -- which include both local weather/environmental "imminent threats" and missing-person AMBER Alerts, as well as the hypothetical Presidential announcement of an alien invasion or zombie attack -- are turned on by default. (I believe they also show up on cellular-capable iPads, but I'm not 100% certain about that.)
The only problem with alerts that are on by default is that they don't have much awareness of your sleep schedule or other responsibilities, and as they deliberately use an unfamiliar alert tone, they can be quite startling... especially if a roomful of iPhones all pop an alert at once in the middle of a classroom, seminar or worship service. (I experienced this personally: flash flood warning, training class, several minor freakouts.)
If, on your own recognizance and at your own risk, you should choose to turn off either the emergency or the AMBER alerts, the NY Times pointed out that it's easy enough to do (as did our own Yoni Heisler when AT&T's support kicked in). Simply go to the Settings app, tap Notifications, and swipe down to the relevant toggles at the bottom of the list.
Where's the toggle for the Presidential alerts? Guess what, conspiracy theorists: they cannot be disabled, short of jailbreaking your phone. Clearly, when the time comes for us to be told where to report for our mandatory Google Glass corneal implant/PRISM uplink installation surgery, that's how we'll get the message.
Now please kindly observe this brief public awareness video regarding the wireless emergency alert system. Your cooperation is much appreciated. And noted. On your permanent record. Thank you.