Well the only "Root Kit" is the one recently used on Sony CDs.
It simply installs 2 KEXTs - they arent hidden, they are in the extensions folder with the other KEXTs...
It asks you before installing these though - so unless you put in your admin password to play a CD then you are fine.
the sony KEXTs are called "PhoenixNub1.kext" and "PhoenixNub12.kext".
Other than that i dont think there are any other known root kits.
If there were chances are you would have had to put in the admin password to allow them to be installed. Basically, anyone with crap like that installed on a mac - its their fault for authorising it...
"hey can i install crap on your mac?" "sure let me just authorise it"