the dearth of virii (viruses?) targetting OS X is something i've been very surprised by. while this article doesn't really answer any questions, it is confirmation that we're talking in the ballpark of 70,000 targetting windows, 50 targetting the classic mac os, and a big fat 0 targetting OS X. now maybe that's just a version of security through anomyniy/small market share, but i suspect it's way more than that. frankly i have a nagging feeling this statement: "Indeed, a Mac has no more inherent security than a PC" is patently untrue -- and i bet this is a major reason: "Apple ships OS X with most of the services hackers typically use to invade a system, such as Web hosting, blocked by a built-in firewall. Although experienced users who need these services can easily turn them on, this Apple policy protects the average Mac user from exposure to the most common attacks. While Windows also has a built-in firewall, by default it has been turned off."

now if you're really interested in more info, the discussions on this book review at slashdot give a pretty savvy answer.