A quick Google looking for OS X security issues...

I found reports of just one Mac compromised.

A similar search for Vista reviled "58,000 PCs running Vista were compromised"

Granted, my search was far from scientific but I do seem to be having problems finding real world example of OS X security breaches. Maybe someone more Google literate than me can provide more. Until there are more "in the wild" reports Apple will just go on behaving as it does now.

The obscurity argument is not playing as well as it used to either, with Vista's market share in the US at 18% and Mac OS X 7.94%

Just to be really anal about it, here are the sources...

http://www.computerworld.com.au/index.php/id;128348660;fp;16;fpid;1

http://www.louisgray.com/live/2008/08/i-got-mac-os-x-virus-and-infected.html

http://www.computerworld.com/action/article.do?command=viewArticleBasic&taxonomyName=macintosh_os&articleId=9106098&taxonomyId=123