I figured the hackers around here would understand that these things are more a function of popularity, with one of the variables being the core security design. Thinking you're "safe" using any popular software is pretty much living in denial.
Software systems are so incredibly complex and market forces and technology changes require lots and lots of new code to be written. Today we can secure a lot of problems discovered as harmful "yesterday", but every new day brings new clever tricks that chip away at our old assumptions. It's a cycle that no one is immune to as long as technology keeps moving.
I didn't suggest that mac's were immune to threats, that would be stupid. Given their BSD base, though, mac's are inherently more secure than windows boxes.
I think the difference is great enough that you can't use virus penetration as a measure of system popularity.
In that case, I didn't suggest that virus penetration be the metric, but attempts at penetration.
Although it is likely that there is a correlation between attempts and the perceived vulnerability of the system, you would also expect a correlation between attempts and expected total success (which goes up with market share).
Can't we gauge this by the number of viruses targeted at the Mac?