I take issue with the idea that security through obscurity is in fact any security at all. I take issue with anyone who throws that canard out without knowing all the nuances. Security through obscurity can be useful, as one more stumbling block to break a system, slowing down attackers, or for raising the bar for the talent necessary to break the system. It isn't something to be absolutely relied on in the end, but its use is not automatically invalid.
The argument that the number of installations is smaller creates security through obscurity is invalid here. 50 million vs. install bases as small as about 100 that have been targeted by malware. Apple is the third most valuable brand in the world, even ahead of Microsoft. Their operating systems (OS X and its brother iOS) run almost everything behind that brand. That's not obscure.
I will not consider iOS to be relevant in a discussion about desktop operating systems. Now, Apple holds approximately 5% of the world's OS share. They hold undoubtedly more in the U.S., and I believe that most malware comes from outside the U.S. (Whether they target the U.S. or not is unknown.)
Whether security through obscurity is achieved depends on what a hacker wants to do. If they're looking to create botnets to launch DoS or DDoS attacks (or mass-mail spam, or whatever) they aren't going to target OSX. They aren't going to target Linux, or Windows 7, or Windows Vista either. The largest target remains the millions upon millions of pirated Windows machines overseas, particularly in China. Microsoft does not allow these machines to be updated. They remain vulnerable and will continue to be the biggest target.
http://news.cnet.com/8301-27080_3-10444561-245.html?tag=untagged
Some there speculate that Windows Vista and Windows 7 have a more secure codebase than OSX. I don't agree with that. An updated operating system will almost never be attacked because it's a hard target. In the world of today's computers, as it turns out the softest target (unpatched Windows machines) is almost certainly the largest target as well.
By the way, I would like to thank you for being civil to me. I usually deserve it (not always) but in the nearly five years I've been here I can't remember you being rude to me. I thank you for that and wish your compatriots would follow your lead. I am not against you regardless of what it may seem like.