I posted this post in another thread, but I think it suits here as well. This is just my opinion.
We are currently witnessing some real effort of malware designers for Macs. And I think the whole big "hoo-har" over the Mac Defender all spans down to the fact that if it was a Windows malware program for Windows, the user's antivirus would have picked it up and nothing more would have been said. Unfortunately, OS X systems rarely have anti-viruses installed on them, and the Mac Defender was able to slip through OS X's rather dire malware detecting system and infect systems.
As much as I hate to admit it (I always love saying smugly that Macs don't get viruses to my Windows using friends,) we are seeing a large interest in OS X and malware. Even Apple are panicking slightly, hence the quick update enhancing OS X's malware detecting system. What we have now, essentially, is an Antivirus integrated in the OS. I think we will be seeing a lot more of this malware for OS X in the future. It's happened. This is what I'd call widespread, and I don't think it's going to stop anytime soon.
Apple now have a choice. Admit that OS X isn't as squeaky clean as they like to, or lock down OS X like iOS (but run the risk of upsetting a lot of X users.)
DISCLAIMER: By virus, I mean all types of malware. I know OS X cannot get a true virus, but such occurrence is also rare on Windows now. I was using virus in the generic sense, like a layman would.