At around 7 p.m. ET on Monday, three of the Internet's 13 "root servers" -- the computers that provide the primary roadmap for nearly all Internet communications -- came under heavy and sustained attack from a fairly massive, remote-controlled network of zombie computers. These are machines infected surreptitiously with programs that allow criminals to control them remotely. The zombies were programmed to try to overwhelm several of the root servers with massive amounts of traffic.Link to full text. From the reports I'm reading now, there appears to be no evidence of damage.
Among the apparent targets was a root server controlled by the Department of Defense Network Information Center. There is also evidence to suggest the attackers targeted the servers responsible for managing the stability of the ".uk" and ".org" domains.
A number of technologists I spoke with who helped defend against the attack said it's too early to say definitively where the attack came from, but this perspective from an operator responsible for maintaining one of the root servers suggests that South Korea, China and the United States were the biggest source of computers used in the attack
Krebs also covered an incident of similar intensity and scope in 2002, you can read that report here: Link. BBC coverage here: Link. See also this related New York Times report by John Markoff: Link to "VeriSign Moves to Address an Internet Security Problem."