There is a significant difference between the situation which you describe (testing Mac vs. Linux for hackability—most likely at a conference of some sort) and the presence of a Mac virus “in the wild”
At least from what ive read, there are no current Mac viruses in the wild. An organized teat or challenge at a developers conference don’t qualify.
Of course, at some future point in time theres no guarantee that a malicious hacker could unleash a virus into the wild but Apple would be getting a patch out a whole lot quicker than Windoze ever has. But developing effective Mac viruses is a lot of work. Unlike Windoze systems, this isn’t something which can be done by any beginner script kiddie.
And the other factor is the small market share occupied by Mac OSX. Those who write viruses get a much bigger bang for the buck if notoriety or criminal activity is their objective.
As market share increases, this may not always be the case but for now Apple systems just aren’t first on the target list.
And my own personal opinion is that iPhone users are a much bigger target just based on numbers alone. I don’t have anything specific upon which to base that opinion but it just seems logical to me. Those who are more sophisticated about computer tech can comment upon how accurate that opinion is :)