It seems to me that anti-virus would be a waste of time in a well designed system. Binaries should be protected from modification. Applications with built-in VMs (like browsers) should be secure and with separate memory protection (like Safari). If a vulnerability is discovered in one of these puzzle pieces then the correct solution is to patch the vulnerability. The patch should be provided with the same speed as any upgrade to anti-virus signatures. And if you don't patch a major vulnerability in time... well all bets are off anyway, you can't be sure the virus didn't disable your anti-virus anyway, so you're screwed in any case.
I don't believe I've ever got a virus on my Mac. When I tried to help friends out with their malware on Windows, anti-malware software did a poor job. It didn't prevent infections, and couldn't repair them. My conclusion is you have to stop them at the border with good system design, not with band-aid anti-virus anti-malware.