Phishing is different to a virus and I agree it doesn't matter which computer people are using if they are daft/naive enough to respond to a phishing exploit.
For many (most?) viruses on the PC, the user generally did not have to do anything more other than navigate to a dodgy website or open a dodgy attachment. Which is what everyone does in order to read their emails for example. And, why sensible people use an up-to-date anti-virus package on their PC to pickup and intercept viruses.
But in the case of the Mac, the OS is sufficiently and inherently robust that there is no vector via this route for a virus to be picked up. Until Microsoft started patching their operating system/browser/office software (delete where appropriate) the PC was wide open.
Which brings me back full circle to my recommendation not to have the preview pane open since , if your PC is not up to date with its anti-virus software, the very act of displaying it in the preview pane opens the email and lets the nasty in.
I agree that there are some holes in the mac OSX but these are mainly of academic interest, not exploitable to the same extent that a PC virus is and not out their in the wild. I do wonder how vendors for anti-virus, anti-spyware software on the mac actually survive? What viruses are they guarding against? None known! Maybe it's because they are selling FUD