An article from the always interesting site F-secure: News from the Lab November 1, describes how some music companies use rootkit-based DRM systems to restrict the ability to make copies.
As this particular DRM technique per se is not malicious it is not a problem, but it can generate false positive alarms when scanning with AV software. Further more it can be abused by malware to hide files!
Technical details, link from the blog
Argus