I realize there is a huge difference between a virus/trojans/worms that target the OS and those that target specific applications and services, but the vast majority of users don't care about what kind of virus they have once they realize their MP3s have all been overwritten or that their hard drive has become an FTP drop box for pr0n. They will undoubtedly blame the operating system since the virus doesn't affect Windows. Given the huge range of 'web enabled' applications running on the Mac, I see a day soon when 'OS X viruses' do start to appear. The obvious efforts of Apple to simplify the firewalling process give me little confidence, given that I have yet to see one outgoing request get stopped by it.
For example, on my own Mac, I have PHP 4.3.10 installed and running -- in fact it was running from the day I bought this machine (along with Perl and Python, and probably several other scripting languages I don't use or care about). If you are a budding programmer, this is amazing since it means you don't have to compile or install a thing. But this version of PHP still has 'multiple vulnerabilities' according to Secunia.org. As a PHP programmer, I know the risks are tiny, since I do all of my own coding and I don't use my box to serve anything to the web. But I can imagine lots of other users loading all sorts of self-installing web applications onto their boxes without the slightest awareness that they are exposing their machines to danger. Load on PHPNuke or some other OSS content management system, you have added another layer of vulnerabilities. Add some extenstion and you are down another layer.
As for OS X's 'inherently stronger' permissions... Every week I read more about Linux exploits that 'escalate permissions' or install 'rootkits', phrases I had never heard of before I moved to Unix. "Stronger" is not "impervious". Yes, Windows is a much bigger target. Yes, it significantly easier to attack. And, yes, it takes little more than a cut and a paste to build a virus that can take down a few thousand Windows machines. But I am willing to wager there are a few serious crackers out there working on breaking your Mac right now, just for the credit of being able to say, 'I was the first.'
Don't get me wrong. I left Windows specifically because of Microsoft's half-baked approach to security (the GDIPlus.dll vulnerability was the straw that broke my camel's back). I feel immeasurably happier and safer with the Mac. But to suggest even for a moment that OS X is 'safe' in any concrete sense is to speak words that will surely come back to haunt you.