In 16 years of using Windows with no Anti-Virus software installed, I have never gotten a virus. Maybe Microsoft should do a better job of educating users on what not to click on.
I run a company that does IT Support for businesses. Once a week or so, I'll have a CEO bring in his kids Laptop with a virus and a good number of times it was caused by their kid using some p2p file sharing program and downloading and running cracked versions of software which contained the virus. This is not yet as big of a problem for Apple as the number of software titles in general is much smaller and thus people downloading cracked copies which might contain a virus is going to be smaller.
Two cents.
The recent mac defender showed how simple it is to social engineer mac users because they think its secure.
If you are worried about security , don't do stupid stuff
At least currently, that means targeting Windows users - solely because there are so many more of them than Apple users.
In my personal spectrum of concerns, having my machine occasionally violated by a virus is not nearly as disturbing as full multi year search and browsing histories being stored and shared without my permission or online profiles being consolidated into a single identity. I'm not saying that google does all this - but it's a lot more scary than a virus.
I read most of these cases were intended to be jokes.