http://ask.slashdot.org/article.pl?s...52&mode=thread
http://slashdot.org/article.pl?sid=0...05&mode=thread
http://ask.slashdot.org/article.pl?s...52&mode=thread
http://slashdot.org/article.pl?sid=0...04&mode=thread
http://computer.howstuffworks.com/in...astructure.htm
Some of those are outdated, but you can at least skim through them and learn a little.
Also, I seem to recall some discussion on the physical security at various root DNS servers, and what would happen if terrorist, say, attacked them (by bombs and such, nothing as fancy as an internet virus, or a DDOS attack). Basically, the WHOLE internet may not go down, but a large portion would, or there would be a significant slowdown, and only a limited number of people would have access.
Stephen