In some ways I'm of two minds about this approach. It's true that I don't allow any Windows boxes on my home network so having a Mac-only solution makes sense. However, by not scanning for non-Mac viruses it's possible that your Mac might unwittingly pass along a virus or trojan by email, etc. I run an Enterprise version of Sophos provided by my University and I've been surprised by how many Windows virus signatures it has picked up on my machine from various downloads.
iAntiVirus is a free download, but virus definitions and updates are $29.95 for one year.