Here is a decent, short article about rootkits at EWeek.com. The author takes a realistic and pessimistic view of defenses available now for dealing with rootkits. It's somewhat scary.
The best part of the article is the links to two PDFs on the subject. One is text, the other is a slide show to go with it. I recommend these for anyone who still doesn't get what rootkits are and how dangerous they are.