The most debated factor in the Mac's relatively low malware (Including spyware, viruses, worms and trojans) count versus the waves of Windows malware has been market share. With Windows market share in the 90 percentile, and Mac OS only in the single-digits, it makes much better business sense to infect the larger crowd.
The paradox of this, however, is of legitimate software on the Mac, especially indie developers. The ratio is so disproportionate, with only a handful of Mac malware, at most, yet a sizable and healthy legitimate software ecosystem, enough to make things like C4, WWDC, and a second hall at MacWorld SF possible.
The answer, I believe, has been partially stated, both by Wil Shipley, and by Ian Betteridge. The market forces for software and malware are different, and while the Mac healthily supports the former, it offers no help on the latter.
Market share is a factor in Mac OS's security. The pundits are right. However, market share still doesn't matter when discussing security.
In Wil Shipley's presentation, he argues that by its small nature, the Mac Market presents itself a great opportunity simply because most programmers target Windows, so that there's more users per programmer on the Mac side, and less competition and knockoffs. Similarly, Ian Betteridge also mentioned issues that help the small Mac programmer, of word of mouth, and blog postings. Ian goes a step further, in that word of mouth doesn't help the malware propagate. (Try this new virus! The infection interface is slick!)
What hasn't been mentioned, that further helps the Mac, is of the differences in exclusivities. That is, typically, a user has only one type of software. If they have a good newsreader, they're unlikely to want to use a second one at the same time. The user will have one type of software to the exclusion of others.
This hurts the windows software market, where the plethora of software means competition is fierce, whereas the Mac has many more unfilled niches. But with malware, it's possible or even likely to have multiple infections, all working side by side. With no such exclusiveness, the fact that there's many similar malware programs already out there doesn't affect infection rates. In short, the tiny Mac market is a boon to the indie programmer, but hurts malware. The huge Windows market raises the barrier of entry to the indie programmer, but only aids malware.
Mind you, there are other factors than market share. The Witty virus is my favorite anomaly, since its target was users of the BlackICE firewall. The installed user base of this software was a paltry 12,000, far below even the most pessimistic Mac headcounts. By the virtue of pure numbers, the Mac, while a smaller target than Windows, should be many times larger than a single firewall program. However, Witty also seems to be a throw-back to the ancient days when viruses caused damage directly to the computer's data, instead of silently lurking and collecting identity theft information.
Whatever the cause, and however significant a factor market share is, paradoxically, it's completely irrelevant in discussions of security. These numbers won't change significantly in the foreseeable future, making all conjecture of "What if the percentages were reversed?" purely academic. This factor teaches us nothing about security. What would the moral of the story be, "Don't have a large user base"?
There's more important factors to investigate. The mere fact of market share has secondary effects, ones that we can learn from, should gain our focus. Market share is part of the history of computing, and what arises from the history can be very educational.
