To the best of my knowledge, there is (at least at this time) pretty much no reason to buy anti-virus software for OS X. The exception is if
you are professionally or personally responsible for exchanging a lot of files with PC-using colleagues; in that case, to avoid becoming an unwitting conduit for viruses and malware that affect Windows machines, it may be polite (or required by your employer) for you to run AV on your Mac. There's a solid free option (ClamXAV) and several commercial packages out there.
Of course, that recommendation may change in months and years to come. Just this week, AV vendor Intego has pointed to a presumed piece of Mac-compatible spyware; Mac OS X Hints links to the methods for spotting and removing the "PremierOpinion" app. OS X is quickly becoming more attractive as a target for exploits, plus browser exploits continue to be a concern. But for right now? It's not clear that the benefits of AV apps for Mac outweigh the potential stability and performance penalties.
If you do feel you must use AV, TUAW posted a nice run-down of peaceful antivirus tips a few years ago, about the same time that Apple pulled some obsolete antivirus knowledge base information (carried over from the 'Classic' Mac OS 9 era) from its support site. As some of my IRC buddies point out, we've never heard of any Mac AV software that has caught an OS X virus before it became a problem. Installing security updates soon after they come out is probably better than grabbing antivirus software, in my humble opinion.
Love & snuggles,
Thanks, Rincewind, drudge, proq, Joachim Bean, and everyone else in #ipaddev