Well, it looks like MacOSX users are going to have to keep on thier toes a lot more with security updates as they seem to be getting more frequent :-
http://www.carrel.org/dhcp-vuln.html
This one is pretty serious.
It stands to reason that this will happen, as MacOSX is unix based - take a look at http://www.linuxsecurity.org/ and see how often security advisories are posted !
Best line of defense aside from patching, a hardware firewall.