There have been a variety of stories recently about MacDefender, and the recent [small] wave of malware attacks on Mac OS X systems.
For years, one of the most-touted features of the Mac was its [relative] immunity to malware: the number of viruses, trojans, etc for any edition of the Macintosh is remarkably small: with System 7, it was on the order of scores, not thousands as with DOS/Windows (at the time).
I do not run antivirus software on most of my machines (virtual and physical), because I take a variety of other precautions. The glib statement that Macs are immune to attacks is becoming, sadly, more-publicly-known to be false.
It is interesting to note, though, that this particular attack still relies on poor user understanding, rather than a flaw in the platform itself: it is still up to the user to install this bogus piece of crap, rather than it materializing automagically via a browser plugin, email attachment, etc.