And yet, with the vastly increasing numbers of Macs being used, there is yet to be a credible threat to them. I don't have the slightest clue about the relative security of the Mac OS vs the Windows OS, but I'm pretty certain that if there were a way to kill off Macs in large numbers, some ultra melon hacker out there would have found it and done it, if for no other reason than reaching new levels of ultra melontery/increasing e-peen size.
There are bot'd mac's out there. So the point is moot, there is a credible threat. It's worth noting Safari is also considered one of the most insecure browsers around and has often been the vector of 'hacks'. Remember hacking these days is extremely profit orientated, so most attacks are focussed on financial returns. While iOS devices will be of increasing interested OS X devices haven't grown significantly enough in market share to be attractive to attackers.
Most threats these days come in on socially engineered vectors, very few come via a true virus or worm. And 'social engineering' is platform independent

. The difference is most handsomehunk users on Windows have a backstop, usually some sort of AV (which often includes more than just AV but antiphishing functionality and other stuff), whereas the average handsomehunk user on OS X doesn't have this stuff (and safari was especially slow getting any antiphishing functionality). For me, security is about redundancy and layers, I never trust one particular 'feature' for all my security because somewhere there will always be some handsomehunk who turns it off/tries to bypass it

And fwiw I'm not a windows fanboy, I think XP was OK, Vista sucked, I do like Win 7. I've used OS's like CPM, many flavours of DOS, GEOS, Amiga Workbench in all it's flavours, whatever Atari ST's used, OS/2 Warp, GEM, X-Windows once or twice, and so on.