Wait, a what? Virus scanner? Macs? Viruses? Me fail English? That's unpossible!
True, the viral risk is mitigated by several new and old features of Mac OS X. By default, files created in a user's home directory are not executable, which helps to lower the risk of infection, and applications in 10.5 now take advantage of code signing, helping to prove that they are who they claim to be.
By default, downloaded files are dropped into a new Downloads folder in a user's home directory, which helps to keep suspicious files in one easy-to-scan location. Previous versions of the OS warned a user when opening an application for the first time; now, 10.5 applies that scrutiny to all downloaded files, pointing out when they were downloaded and asking if it's really, really okay to open them.
Even with all of those extra measures, no system is immune to malware, worms, or viruses. Some of the measures described above would prevent another Leap-A or Opener from spreading too widely, but until every hole is closed and every bug is eliminated, new threat vectors will arise. (In other words, forever.)
Given the sheer pervasiveness of Microsoft Office in the Mac universe, it's impossible to ignore the specific threat of macro viruses. While they can't really do any significant damage on the Mac side—especially with the recent demise of VBA scripting—they can be passed along to fellow Office users on, shall we say, other platforms. And guess what happens then? Hopefully the answer is "they get caught by an anti-virus program," but the point remains.
If it bothers you that we have to take steps to protect ourselves from evil that can't really harm us, consider it a non-deductible form of charity towards our well-meaning but misguided fellow computer users. (That's a joke, people, stop trying to launch Outlook.)
The Mac OS X version of ClamAV
Although there are several commercial virus scanners for the Mac platform, I personally prefer to rely on a tried-and-true open-source alternative: ClamAV is available in a specially-built Mac OS X version and receives just as much (if not more) attention as the for-pay alternatives. By setting it to auto-update and keep a close watch on your Downloads and Documents folders, you're already several significant steps up the security ladder.
http://www.clamav.net/lang/en/