@ Robert Grant
You said, "rootkits are bloody difficult to deploy compared to how easy Windows exploits have always been." Exploits are usually the path via which rootkits and other resident malware is introduced to a system, regardless of the OS. Just check your Unix-based system's SSH logs and you'll see a lot of automated attempts to log in. These are, in my experience, part of an attack that will attempt to drop a rootkit onto the system. In light of that, "you have to do so much to deploy a rootkit you may as well go create a Windows virus" does not make much sense.
Regarding the 'root' account: as we are playing with words here (root -> rootkit), it's only fair to point out that Windows does not have a default account called root. The administrator account is probably what you are thinking of. Mr Miles' point about the etymology of the word 'rootkit' still stands, to my mind, and further support can be found on the net. For example:
"The term rootkit (also written as root kit) originally referred to a set of precompiled Unix tools such as ps, netstat, w and passwd that would carefully hide any trace of the intruder that those commands would normally display, thus allowing the intruders to maintain root access (highest privilege) on the system without the system administrator even seeing them." [Wikipedia]