Why is Mac OS X virus-free ?

Perhaps you should specify that you refer to Mac OS X. Before that, I remember some (not many) viruses (nVir...) we had to deal with.

The subject of "computer security" is a large and complex one. The subject includes viruses but viruses and security are often wrongly seen as the same thing by the press and less techie folk and much confusion arises. The fact a computer is highly resilient to viruses does not mean it is highly secure - or safe for the average user to use.

What Windows managed to do was to make a viruses a very popular way of compromising computer security. They are understandably a huge worry for all MS PC users and are seen as "fact of life". the fact so many people have PC's its understandable folk think its a computer issue instead of a Windows issue. Yes, no one can prove that the Mac is invulnerable to viruses and perhaps its only a matter of time before we see them (who knows?), but the fact remains its just different for macs. If you want to make money/notoriety in being naughty with mac based computer networks, virus creation is just not an easy (and hence "the best") way to do things. I suggest a much easier route is to go email phishing for example.

One thing PC users have over mac users is a heightened (though perhaps specialised) awareness of computer security and thats thanks to viruses - and that can only be a good thing! If you are going to worry about viruses you should also worry about other issues too like unprotected wireless home networks, leaving passwords written on pieces of paper and countless other "exploitables" that plague our high tech world. Thankfully with a Mac you don't need to put in as much effort into fighting the virus element of things.

Viruses are becoming more and more "so yesterday" IMHO.

True, the Macintosh operating system with it's UNIX DNA is rock solid and resists malware in the broadest spectrum of known threats. However the Mac and it's os comes with bundled software modules and therefore bundled vulnerabilities.

True, Apple has a response time that sets the reference for the industry.

Not true, hackers do not write malicious code aimed at the lion share of the market. A feasible chance of success strangely attracts them, regardless of market share. Cross platform applications by nature have an Achilles heel, they need to be patched platform specific released at the same time.

I have to agree with the point Thomas brought up about awareness. Stressing the Mac's outstanding safety track record as a unique selling point might undermine user attention regarding the need of safe computing.