As a related note, Google.org has launched a project called Innovative Support to Emergencies, Diseases and Disaster (InSTEDD. InSTEDD does not have a live software product yet, but organizers have said that it plans to use, among other things, a mash-up of SMS alerts and Google Earth. (They also have begun to maintain a rudimentary flu-tracker: http://www.google.org/flutrends/)

My county currently employs the SMC alert system from http://www.coopernotification.com/ Its a free, public emergency alert system, and many other US cities and counties utilize their service as well - a list of them is here: http://www.roamsecure.net/content.php?contentID=50

These services depend on SMS and email being available.