Some things, like clicking on a link and opening an email may not be safe; visiting a web site; an unpatched vulnerability in Flash, Java, or other plug-ins, those still exist.
If you see a pop up that tells you to click on to scan, or alerts you that "your system....." it is malware. Ad servers can be infected. Google has had to try to clean their cache system.
But virus in the traditional meanings of malware/spyware and rootkits or that AV traditional looks for...
If you find that your email program doesn't filter junk or spot phishing exploits, you may, especially if you share files, or your ISP would hold you responsible for forwarding an email that while benign on OS X could affect the recipient's system.
You didn't mention that the BBC thought it was UN-necessary:
Eugene Kaspersky, co-founder and head of anti-virus research at Kaspersky, predicted a "significant rise" in Mac viruses back in April 2007. Has it happened? Not yet.... Symantec and McAfee both sell anti-virus software for Macs. So does anyone have any anti-virus software installed on their Macs? I don't on my machines. So am I smug and stupid or smug and wise?
http://www.bbc.co.uk/blogs/technology/2008/03/macvirus_alarm_is_soundedaga.html
Now, do I own but not use AV? Yes. I want to have something in place, if it is needed. ClamXav, Intego both offer programs or suites. Safari is not my browser of choice by default, but it does log errors, and the iPhone might be the most likely portal for a worm or virus, someday.
