This is an overly basic Q. Several Mac users tell me they don’t use any antivirus or other protective software at all because viruses & malware aren’t aimed at Macs. Is this true, or how accurate is it?
There is some Mac Malware. Mac users certainly need to watch out for ‘social-engineering’ attacks; whether or not they need AV protection is a moot point.
http://sunbeltblog.blogspot.com/2007/10/mackanapes-can-now-can-feel-pain-of.html
Absoutely NOT, whilst there are few viruses/malware specific to Macs as FWF said there are aspects you have to take care of.
As .: Mac :. a forum member keeps trying to tell us that the Mac market share is increasing. So will the attention it gets from the malware writers, currently it would appear it just isn’t worth their time because that market share is still small.
The vast majority of Malware is aimed at windows. And Most Mac users do not run antivirus software on their systems because of that fact.
But The Mac OS is far from immune from attack, there is malware for Mac OS X out there (Such as the DNS Changer Trojan) its just not that widespread. But as the Mac increases in market share it may become a riper target for hackers to focus on. So just because there are very few malware programs out there today, doesnt mean there wont be 5 years from now but so far the Mac OS has held up pretty well.
There are some Antivirus programs for Mac OS X for the users who feel like they need to run one, Avast even offers a version for Mac. But unless the number of malware programs for the Mac increases dramatically the majority of users still wont use AV Software, because most users switch to the mac to get away from the hassles of windows such as having to worry about Malware. Put simply, Mac users want to spend their time enjoying their computer instead of spending half of the time on maintenance. 
Also as FreewheelingFrank pointed out Social-Engineering attacks can affect users of any Operating system (Mac, Windows or Linux) so always be on the look out for those.