Many of my clients ask, "What kind of anti-virus software do you use?". They are inevitably surprised when I say, "I don't use any". This does not mean I don't take precautions to protect myself online, on the road and at home. I back up 3 ways: Time Machine, a cloned back up and with an online back up service.
Here is an excellent article on TUAW (The Unofficial Apple Weblog) on the subject by T J Luoma:
Securing Your Mac - A Guide for Reasonable People, Version 1.0