I'm pretty sure this article and the pawn2own conests have proven it's not inherently secure. The only thing that is in debate is why the virus writers haven't put much effort in attacking OS X.
I say it's because it's too small of a footprint and is secure enough to keep their efforts at exploit limited to the dumb users. For those attacks it's best to focus on the >90% of the market.
I'm not sure what you're saying though. You are saying it's inherently secure when we know that's not true based on pawn2own and this article detaling all the holes in the system.
They have put in the effort. Not much real-world success so far. I believe it is inevitable that one will eventually be successful, but even one out there puts you in far better shape than on Windows.
I say it's because it's too small of a footprint and is secure enough to keep their efforts at exploit limited to the dumb users.
But you don't explain how populations smaller and more obscure than that of OS X have been targeted with wild success.