Yes, Macs running OS X are spared the worry of viruses…but don’t forget that under that attractive desktop interface runs Darwin (a variant of OpenBSD unix). Here’s a document that goes into very good detail on securing any Mac you’ve attached to the internet. Many recommendations probably go beyond what a desktop user typically worries about but if you have a need for enhanced security, this pdf from Corsaire (a network security solutions company) will show you how it’s done.
Author: Stephen de Vries
Title: Securing Mac OS X 10.4 Tiger
Date: 19 August 2005
Download from Corsaire’s site
[locally cached copy]