The best way of protecting your mac is using the "principle of least privilege".
I find there is no point in having a Mac anti-virus, if you use a standard account for your daily computer activitys and use an administrator account for installing things on.
On a mac if you dont hold enough privileges on the standard account you can use admin credentials(if you have them of course) to still do stuff you want while minimizing the amount of harm a malicious app can do on a standard account.
With the upcoming Mountain Lion it will be even harder for these malicious apps to do anything at all without getting a legit software developer ID.