Look, I keep on top of the malware issue for the Mac. I maintain the Mac ping list on Fr. I post every damn report of them on FreeRepublic when the come out. Don't try to tell me what there is out there for there in the wild when you don't know beyond the hype you read.
Currently, puget, in the wild, there are ZERO known self transmitting, self replicating, self installing viruses or worms for OS X. There were about ten known in the lab candidates for such viruses and worms that have been announced and given names such as Renepo, Macarena, Leap.A or Oompa-loompa, Inqtana.A & B, etc. These were three day wonders in the press and were announced as "the first Mac virus" and "Apple users will finally have to use Anti-virus ware!". . . But they were ALWAYS merely proof-of-concept virus that DID NOT WORK! and were never seen outside of a computer security company lab. One of them, as an example, took the dilligent efforts of two Apple engineers, two Apple security specialists and two reports from Macworld magazine over six hours of effort just to get it to do what any self respecting virus is supposed to do: copy itself from the infected computer to an yninfected computer. Once they succeeded in doing that, it didn't run on the invaded machine because it didn't have permission! Inqtana, when it was announced took advantage of a Bluetooth vulnerability that had been closed for over a year... And STILL required the user of the invaded machine to accept and run the invading file. Some virus. Knocks on your door and asks permission to come in. Then you have to run it for it to do any damage.
On the other hand, there are approximately fifteen known malicious Trojan horse programs out in the wild, in three distinct families. They require a distinct effort and stupidity on the part of the user to download, install, and run, ignoring red flags and system warnings built-in to OS X. These are easily avoided. If a user cannot be trusted to practice safe computing, even running an anti-virus app will not save him by adding more warning on top of those already provided.
So, Puget, until a credible viral threat arises, there is not much reason to run anti-virus software on a Mac. In ten years of trying, the virus writers have yet to find a viable vector to transmit their malware.
OSX/Leap-A was a real, in-the-wild virus from 2006. Apple itself recommends virus scanning software because viruses and trojans exist in the real world, in the wild.
On the other hand, there are approximately fifteen known malicious Trojan horse programs out in the wild, in three distinct families
Which is the case with roughly 99.9% of all "viruses" in the Windows world - trojans. I don't run a virus scanner on my daily computer (currently a HP G71 that I'm typing on now) and have not had an infection (checked monthly) for 4 years, on three different laptops.