I'll add that I was doing some research for an internet and computer-less friend about Mac OS X and stumbled upon the little hint in the first paragraph on Wikipedia, which subsequently included the little link number (which ultimately led me to post this.)
In any case, I do have Symantec (well okay, it's Norton) on my Mac mini, and the Dashboard widget shows the latest threat level and what threats are out there.
I don't believe in digital downloads for software, and I really don't believe in Limewire, and I probably never will. ("Get off your lazy butt and go to the store and buy software, you dolt!" to quote an IT guy I know who doesn't believe in it either.)
In any case, it is proactive to use an AV on a Mac- and Dad trusts AVG and Symantec because Symantec catches a lot of things before other AV's do (at least in his observations..he used to use McAfee- and I will say this is due on the government's part, they used to use McAfee and now they use Symantec- he gets a high-end gov't edition).
Does AVG exist for Mac OS X?