That's flawed logic. I understand what you're getting at, but the argument is that there's no point going after MacOS when Windows is the vast majority. 10% is nowhere near a majority, so 0 people who want to attack an OS that is not the vast majority times however many virus creators out there is still 0.
And for those of you saying there's no worry for Mac's, you are incorrect. There is a Trojan out right now that completely takes control of your Mac.
What you don't have to worry about on Mac is something seeping in without you having done anything. The Trojan I spoke of above requires you to install what claims to be a codec. There are almost none of these, but they DO exist. But as long as you're only installing things when you know what they are, you are safe. And even if you install things when you're not sure what it is... the odds are you are still safe. It's analogous to being struck by lightning... there are so few out there, that you're really not at much risk... yet.
I think the point of the author of this thread is, what if Mac becomes even more popular? With the failure of Vista, this is somewhat of a distant possibility. Then Mac becomes a target... no argument that MacOS is far more secure than Windows... but when the incentive is there, trust me, virus's will occur more frequently.