And mine, too, that the lack of effective self-replicating malware in the wild for OS X is NOT because of the low marketshare.
OS 9 had such a poor security architecture that developer tools for it specialized in catching potential memory writes outside of the program's address space. As you know, that's a common method of exploiting a system, and it was easy to do just by accident.
Personally, I'd love to see a full-blown OS based on a true microkernel like Minix or seL4. The latter has even been subjected to a formal proof of its function. That's enough to get EAL7 certified, something no other operating system has achieved.
Well let’s see what the person who actually hacked the Mac has to say:
http://blogs.computerworld.com/15605/hacker_pwn2own_organizer_windows_7_is_safer_than_snow_leopard
And what the organizer of Pawn2own says is in there as well.
They both agree at worst Windows 7 is as secure as Snow Leopard. At best it is better.