I always suggest: Just because you don't use Windows it doesn't mean, you don't have to bother about antivirus software, firewall and system upgrades! Every code, even written under Steve Jobs, is somewhere buggy, every developer will forget about a security issue, and it just a matter of time until someone who is interested in demolishing a system will find it out.
When you care about your system, actually every OS is secured. I just believe, that MacOS is in the moment, the one that you have to care at least about. But that never means, you don't have to at all!

I read an article once (sorry, don't have the link) where it suggested there have only ever been 2 viruses in macs, and in each case the virus was harmless. Apparently there's no such thing as a mac virus at present.

I've had my mac for a good few months now - I browse torrent sites etc on a daily basis with NO firewall/antivirus whatsoever, I so far don't seem to have found a virus. Some would suggest it would be more harmful to install unneeded antivirus software.

Yes, recently Kaspersky announced anti virus for mac users. http://www.beepthegeek.com/2009/11/hi-mac-users-now-get-advanced-internet-protection.html