It's more than CLEAR that pretty much any Mac-related threat is based on social engineering and nothing else. There are NO known viruses in the wild, and there's NEVER been one for OS X for that matter.
Besides, EVEN the latest variants of MacDefender (itself inocuous, by the way) required active intervention by the user at one or more stages, be it for installation or running that software (not to mention that "safe open" should always be disabled in Safari, of course) - and MD did NOT install itself in the root-level apps folder, so sandboxing is even more useful here.
I do NOT use AV and do not intend to do, unless something really changes in the Mac malware environment. Lion is already the safest OS on Earth, and I do NOT intend to bloat my computer with irrelevant CPU/RAM demands.