Here's an interesting bit of news. According to Mark Russinovich's blog, both Alcohol and Daemon Tools use rootkits to circumvent DRM.
An easier approach is to fool game DRM software into thinking its reading data for playing a game from its original CD rather than from an on-disk copy. DRM software uses a number of techniques to try to defeat that trick, but a straightforward one is simply to detect if CD emulation software is present on the system and if so, if the game is being run from an on-disk emulated copy. That's where rootkits come in. Two of the most popular CD emulation utilities are Alcohol and Daemon Tools and they both use rootkits.
It will be interesting to see how the developers of both programs respond to Mark's claims. If you'd like to read more, his entire blog post can be found here.