[chris]

9 of the 13 root servers were taken down via a DDoS back in 2002.

http://c.root-servers.org/october21.txt

Although the report states "2.4. There are no known reports of end-user visible error conditions during, and as a result of, this attack.", it's not entirely accurate. I personally experienced issues with name resolution shortly after the attack started, and had no idea what the cause was until afterward. If I recall correctly, my name resolution was handled by Qwest, as they were the T1 transit provider I was using at the time.