Recently there was a story about malware in a Linux distro, with Ed Bott claiming that the story proved that Linux users need anti-virus programs just like Windows users.
http://forum.avast.com/index.php?topic=19387.msg513495#msg513495
I’ve gone back and looked at the malware concerned and found that anti-virus programs didn’t detect the malware for the seven months it was in the Linux repository, and only 4/41 anti-virus programs detect it now, more than a month later.
http://dontsurfinthenude.blogspot.com/2010/07/in-part-1-i-wrote-about-linux-malware.html
A similar story about malware on gnome-look.org has similar findings.
http://dontsurfinthenude.blogspot.com/2010/07/how-useful-is-anti-virus-in-linux-part.html
Caution in where to install packages from in Linux will protect users from malware, where anti-virus programs won’t.