We posted last December on the DC Text Alert program, which sends emergency alerts via SMS. The Washington Post (via Yahoo! News) has more details on the system, which is now called the Community Emergency Alert Network. Traffic, weather emergencies, road closures, and the like are beamed to the text-capable devices of over 16,000 subscribers. Broadcasts are available in English or Spanish.
You can get similar alerts for New York City as well, by going to http://www.nyc.gov/oemnews.