Virus free doesn't mean there aren't any worms, trojan horses, bots, malware or vulnerabilities in 3rd party plug-ins and media plays or content.
Use your router and common sense still applies. Clicking on a link in an email - I wish companies would stop asking customers to click on links to opt out or check on order status.
Ads still show up, pop up, even though they aren't really ads at all, but malware and attempts to gain access or identity theft.
The trouble is, ClamXav doesn't spot infections in Office files, and can only delete a file rather than repair it, and the few companies claiming to have products don't have much practical experience or R&D, even as malware and bots have grown and will only get worse.
I don't feel any safer on Mac than on Vista.