Is truth in advertising a myth where Apple is concerned? Judging by the clever manipulation in Apple's current Mac OS X slogan, I'd have to say yes. That slogan - "Mac OS X doesn't get PC viruses" - could just as easily be twisted by Microsoft to read "Windows doesn't get Mac viruses".
Actually, through Parallels and BootCamp, there's a better chance that Mac could (at least theoretically) be impacted by a PC virus. But on the flip side, PCs don't run Mac OS X so there's no chance of a PC being impacted by a Mac virus. From that standpoint, Apple's slogan is an even bigger misrepresentation of the truth.
To be clear, Mac OS X can't be directly infected by a PC virus - but it can be infected by Mac malware and that's the bit that Apple seems to be obscuring.