The first nationwide test of the U.S. Emergency Alert System (EAS) will take place at 2:00 PM (Eastern Standard Time) on November 9, 2011. According to the FCC web page set up to detail the test, the purpose of the test is to assess the reliability and effectiveness of the EAS as a public alert mechanism.
The system essentially allows the President of the U.S. to seize control of all public broadcast signals to deliver information he deems important to national security, directly to the American people.
For more information on the test itself, here’s a link to the FCC web page on the EAS test: http://www.fcc.gov/encyclopedia/emergency-alert-system-nationwide-test