Eugene Kaspersky, of Kaspersky Lab fame, has published a short article on rootkits.
It's not too technical for most to understand; it links to an interesting test of anti-virus software versus pre-installed rootkits; and it notes the easy availability of rootkits on the black market.
Eugene writes that, "rootkits are not all bad/malicious," but that, "the general public has come to view this technology as a bad thing."
However, he concedes that, "rootkits represent really dangerous technology."
In the past 'legitimate' uses of rootkit methods have been PR disasters. For example, Sony's adventures with rootkits actually made systems more vulnerable to other threats.