One In Five Macs Holds Malware — For Windows
judgecorp writes "One in five Apple Macs is infected with malware, according to Sophos. But most of that is harmless to the Mac... it is Windows malware ready to be transmitted to the Windows population. Only one in 36 Macs has OS X specific infections."
It's always been a good idea to have a virus scanner on a Mac - at the very least, it's a courtesy to users of other platforms who may be more vulnerable to any infectious crap you may pick up without realizing.