In the early morning of 2009-04-09, somebody climbed down four manholes in Morgan Hill, California (near San Jose) and cut a few cables. This essentially "cut off" much of the area's communications, including the local 911 call center, and the INTERNAL network of a local hospital. The city and county got through it with the help of a group of local ham radio operators, who volunteered their own time and radio equipment to set up temporary communication links between the hospitals, the public safety (police, fire, rescue) agencies, and other relevant sites, while the cables were repaired.
Bruce Perens, known as one of the founders of the Open Source movement, and a fellow amateur radio operator, wrote an article about the incident, where he points out how stupid it is for companies, and especially government and public safety agencies, to allow their core functions to rely on outside parties. The idea is that an organization's technical needs should be hosted in-house as much as possible, so that if the internet or telephone lines go down, they are still able to function- perhaps in a reduced capacity, but they shouldn't be totally "down".
Here's a perfect example. One of my clients is a company whose employees make heavy use of "instant messenger" programs to ask and answer questions, provide updates, and transfer files. It works for them, unless their internet connection is down, in which case they can't connect to AOL's servers. Which means that in order to send a message from one office to another, that message has to go all the way up to AOL's servers in Virginia, and then all the way back down to their office here in Florida. Even though they're in offices which are right next to each other. If this company were using XMPP (aka Jabber) instead of AOL's proprietary system, they would be able to host an XMPP server within the building, and all of their IM conversations would never have to leave the building at all.
I just thought Bruce's article was very well-written, and explains the issue clearly enough that people can understand it. If you are even remotely interested, take a few minutes and read the article.
The link again: http://perens.com/works/articles/MorganHill/