OSX is basically a unice with the root account turned off by default. It's difficult to imagine a non-embedded system with less potential for vulnerability, especially considering the quality of patches coming from apple. As long as you've got a strong admin password and are careful about using it, you keep up with the security updates, you backup and you don't do anything really stupid you'll be fine, a virus scanner is a waste of money and computing resources. That might change at some point in the future, but for the moment it's not worth bothering.
