I admit that I do have ClamXav installed on my Mac, but I rarely ever fire it up. I mainly use it to scan files that are sent to me by people on Windows computers (one of which is my wife ).
I've got an antivirus - but that's only for the XP virtual machine on my Mac. Since on Mac OS X side I don't even use Microsoft Office, the likelyhoods of discovering a real virus is 0.
This thread also contains a lot of info http://macosx.com/forums/mac-os-x-system-mac-software/49989-viruses-os-x.html on pro and against