Even if you use a Mac, you can still get bit--HARD.
There are no mac viruses in the wild that should concern any Mac user. And trojan horses are different, as in nothing can really prevent that, except the obvious solution of not giving your admin password to every app that wants to install (just the really trustworthy apps from the original source). And even then, it's usually pirated software that people get bit by on the Mac.
Some key advice is to never give your password if you have no clue what's going on, or why an app, website, etc, is asking for it, always google that app to make sure it's legit and even look for other links (usually found in reviews, etc) to that same site to make sure you're on the right website.
But basically, if you're not running a mac in some type of network, you probably shouldn't be worrying about viruses.
Note that the only three viruses I found with clamAvX were for Windows.
That said, if you're on a Mac, use Littlesnitch. It'll tell you if something on your computer is trying to connect elsewhere, which some trojan horses try to do (that pirated iworks trojan from last year). But being smart with your password and not pirating software is the best solution for that.
Just be smart. If you're on mac, linux, win 7, be careful with your password (
and don't disable UAC on win 7), don't click that link if you don't have to, don't trust shiny, yet cheap looking sites (a personal thing for me), and keep everything (browsers, operating system, anti-virus) updated.
Last note. ClamXaV isn't an active scanner, as in don't expect it to pop up if you do download a virus, and install it. I basically run it once a week or so, part of my general maintenance, to make sure everything is okay (and to not infect a friend who uses windows with a virus meant for them). iAntiVirus is an active scanner.
http://www.tuaw.com/2010/06/03/dear-aunt-tuaw-should-i-buy-anti-virus-software-for-os-x/
http://theappleblog.com/2010/02/04/antivirus-software-on-your-mac-yes-or-no/
http://www.clamxav.com/
http://www.iantivirus.com/ (supposedly very lightweight, and possibly has a free version?)
Edit- I'm now using iantivirus

, since it seems lightweight enough to be no problem. Nothing wrong with being
paranoid safe (i've been burned by years of using windows), even if nothing is out there...