Apart from testing for an incompatibility between a piece of Mac AV software and our software (turns out the problem was with the AV software, what a shock! :D) I've not run AV software on any of my macs for a very long time. The last AV software I ran was Disinfectant.
The only virus I know of that would effect a mac would be Office (not sure if it's just related to Word, or all of the Office products) Macro viruses. Apart from that, there hasn't been a "real" virus on the mac for quite some time. The real reason for running AV software is to prevent you from passing on infected files to a PC user, and if they don't have any AV software installed they are very foolish. I run AVG and Ad-Aware 2007 on my BootCamp XP install, as well as all my other PC's and I've not had a problem.
If you really want a solution for OS X then have a look at
clamxav. It's essentially a mac implementation of ClamAV, which is said to be very good indeed.