The best news is that although it's a worm, it still requires a TROJAN to deliver it. . . and Apple still warns all users of all the known trojans. It would take an entirely new family of trojans to deliver it, because OS X will recognize all the current families and their variants.
You need to read the full details of this hack. Any Mac physically connected to a network can be infected without user interaction. Wired has a much more detailed article on this, but can’t be posted on FR due to copyright issues.