Whether it's a Mac, Windows, Linux, Solaris, or whatever, if it is turned on and online, then it is vulnerable.
As mentioned before, Mac's only make up a small percentage and as such they are less of a target. However as the other OS's are tightening up their security, I would imagine good ole Mac will see more assaults upon their OS. How will they react when the attacks begin. Do the have the experience that the other companies have acquired from years of patching? Some good questions. And of course with Mac and all other OS's, hopefully they'll kick some hacker butt!