I don't want to start fear mongering, but some time ago they said that terrorists could target the powergrid.
From BBC in Oct. 2001:
"Computer experts have painted a chilling picture of the potential threat from a combined terror and cyber-attack on the United States.
They told Congress that terrorists could target computer networks critical to power supplies, telecommunications and financial systems and wreak havoc on the country. "
http://news.bbc.co.uk/1/hi/sci/tech/1593018.stm Then there were recent reports that Al Qaeda has been planning major attacks in multiple countries this week.
So far they say that "there is no evidence that this was terrorism related" and it's a "natural occurrence" (?!).