An attack yesterday on Akamai servers disrupted service for some of the biggest sites on the Internet, including Yahoo!, Microsoft, and Google.
Akamai servers are located throughout the US and overseas and help speed up major Web sites by locating some of the content closer to users. According to the article, an Akamai representative characterized the attack as a "large scale international attack on the Internet infrastructure."
There is a lot of speculation about why it was done, but one possibility to be considered is terrorism. A handful of sites in the world attract a very high percentage of Web traffic (over 90%). If a persistent, determined attack degraded service on major Web sites for an extended period of time, it would have major economic impacts, and the values of many kinds of IT stocks would likely experience significant losses.
Terrorists don't have to do much--they just have to attach a few high visibility targets, and fear, uncertainty, and doubt will do the rest.