Apparently, Sony isn't the only mainstream company that is using root kit technology in their software. According Mark Russinovich, the guy who discovered Sony's root kit, anti-virus software by both Symantec and Kaspersky also uses root kit technology.
Symantec has been using this technology in their "System works" package since 1990 to hide backup files. They claim that it is done to prevent users from accidentally removing the backup files. On the one hand I can imagine and respect that they want to protect their customers from screwing up their system, but on the other hand it also provides a service to people that want to hide their spy ware, etc. Anyway, Symantec released a patch that makes the hidden files visible again.
Kaspersky Anti Virus uses root kit techniques to hide checksum-information. The software uses this information to verify what files have been scanned, and what files haven't been scanned yet. They claim that it is impossible for third-party software makers to use the technology; although many companies made those claims before and they all failed miserably