Recent versions of OSXServer have included the open-source antivirus engine "ClamAV" as part of the OS. The regular version of OSX does not, but what with being open-source you can get it through other means, and a frontend to it called
ClamXAV exists. This frontend has been making quite a few improvements lately, and it now shows a lot of promise. I'm guessing that if Apple starts doing more with anti-virus stuff in OSX, it will use ClamAV rather than Virex.
I suppose this is just as well. Virex was very good on OS9, but it never got to be more than a pale imitation of its former self on OSX.