The odds are very, very small of getting infected with any Mac malware. Therefore, IF you have a complete backup of your hard drive that you update regularly (use Carbon Copy Cloner, or the paid version of SuperDuper!), then you really don't need anti-virus. If you ever did get infected, you could simply wipe your drive and boot from your other drive, then dupe again back to the original drive and keep on trucking. (I would suggest having your computer backup your hard drive daily at night automatically.)
PS I run a PC along with three Macs and I have never run anti-virus at all on the PC and I've never had any trouble. Perhaps it is because I am behind a router with a firewall, then run everything through opendns, then run from that router to a switch, then to the four computers. Maybe the computers are not "seen" on the internet, but I really don't know (and don't care--I don't have ANY crucial data on the PC and only use it for kids games and what not.)
If I am being stupid, please tell me.