Security researchers at Black Hat claim OS X networks are significantly more vulnerable than Windows. In fact, reports ZD Net, OS X networks are significantly more vulnerable to network privilege escalation, and almost every OSX server service offers weak or broken authentication mechanisms.
"Things are looking up for Mac OS X though, as 10.7 ‘Lion’ adds application sandboxing to keep malware contained. Whether this will be enough to make Lion more secure than its predecessors remains to be seen.”