Today (Wednesday) at 2:18pm et FEMA is conducting a nationwide test of the Emergency Alert System. Your phone will likely (and hopefully) air that noise you hear on the radio when there's a severe thunderstorm or tornado warning.
Again, it's NOT AN ACTUAL EMERGENCY, just a test of the national system to make sure compatible phones work. So if you still use a flip phone or blackberry, you probably won't get the message.
The text that comes across your phone should read:
Presidential Alert: THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.
photo: William Hook, Flickr Creative Commons