The thing about Anti-Virus programs on the Mac is this - and I've said it before in other threads - there are no current "in-the-wild" viruses for OS X, period. Not one. There have been "proof of concept" viruses, but nothing that has ever infected an OS X computer unintentionally.
Most Anti-Virus programs use "DAT files", that is, databases that contain data about all known viruses and ways of dealing with them. Those DAT files are updated constantly so that the Anti-Virus product is effective. In addition, some anti-virus products use heuristics technologies to try to determine virus-like activity and stop it.
Since there are no viruses for OS X, any Mac anti-virus product is only going to be as effective as its heuristics algorithms. Historically, heuristics have proven to be mediocre in terms of effectiveness.
That said, any virus for OS X is going to make headlines. If you're on planet Earth at the time, chances are you're going to hear about - even before it makes it's way onto your machine. Windows mavens have been chomping at the bit to quell Apple's claims of OS X's resiliency in terms of security. So when an OS X virus appears, the media will be all over it. Considering what an accomplishment it would be, it's a wonder no one has attempted it.
So, I think it's a pretty safe bet to run your machine without an anti-virus product for the time being. If you install Windows via Bootcamp or Parallels, I'd recommend installing AVG Free Edition to prevent damage to those installations. You should also note that it is possible that the Mac could convey a Windows virus to another Windows machine (without being infected itself). I'd imagine that a Mac anti-virus product would look for files like that to try to mitigate the chance of it infecting someone else's machine.
Just my $0.02. I've seriously considered buying one, but after having given it careful consideration, I've decided to wait and see what happens.