I don't trust this old argument. As if no hacker ever tried. It would make so much noise if a decent virus was developed for mac (last one, Flashback, only infected a few hundred thousand computers, and was removable with a simple executable, or the next java update). I don't think that no decent hacker ever tried it. We're a billion people using internet everyday, there must be legions of hardcore unix hackers in the middle who would find this a very nice challenge. I don't say it's absolutely safe from viruses, but it seems like the way the system is closed, the growing tendency for sandboxing, etc, makes it more difficult. Which joins your argument that it's less profitable. But I do think it's because of the OS itself, not just the number of units especially now that everyone knows someone who bought a 13in MB or a MB Air.
Reading 9gag these days is enough to be convinced so many people would just love to see Apple badly hit in its pride, as well.