I've heard that there is a Mac virus, a trojan horse, showing up in illegal software downloaded off BitTorrent.
Basically, if you don't download illegal copies of software from BitTorrent or other pirate sources, you have nothing to worry about and you can stop reading this post. I think eventually someone will write a nasty virus that will spread via web browser, like we have seen time and again with Windows and Internet Explorer, but today is not that day.
The way the iBotnet Trojan virus installs itself is only through installation of the software, at which point the user enters their password and it unlocks the computer for the hacked copy of the software (and consequently, the virus) to be installed.
Removal information on theAppleBlog.
Mac viruses are practically discussed (by a photographer, no less) today on Ken Rockwell's site.