A couple of news stories, one old and one newer, highlighting how vulnerable the system is due to its dependence on complex infrastructure:

Saboteurs Knock Out Telecom: Saboteurs knocked out land lines, cell phones, Internet, ATMs, credit card readers for police, banks, businesses and residents in three California counties on April 10, merely by cutting 8 fiberoptic cables. “In each case, the vandals had to pry up heavy manhole covers with a special tool, climb down a shaft and chop through heavy cables.”

U.S. Energy Grid Vulnerable: A group of retired military officers has issued a report warning that the U.S. military grid is highly vulnerable to accidental or deliberate disruption. “Our domestic electrical system is also a current and significant risk to our national security: many of our large military installations rely on power from a fragile electrical grid that is vulnerable to malicious attacks or interruptions caused by natural disasters,” the report reads.