The bad guys (namely, the Russian Zlob gang) have been paying attention to Macs intermittently off and on for some time. As far back as four years or so ago, I found some fate Windows antivirus sites which would look at the browser user-agent and download the OS X version of the RSplugin/DNSchanger malware if they saw a Mac browser, and a Windows malware package if they saw a Windows browser. The fake scareware sites still looked like Windows antivirus scans, but it shows they were at least thinking about Macs.
The thing they still haven't done, and something that Russian organized crime has offered a reward for if anyone figures out how to do it, is to install malware without an administrator password. On Windows this sturns out to be pretty easy to do, but after considerable effort, organized crime still hasn't managed to do it on the Mac yet.
A lot of folks claim that Windows is overrun with malware and Macs aren't because Windows is more popular. The reality, of course, is that the Mac really is a harder nut to crack; figuring out how to install malware on Macs would be really profitable, but that damn "can't make it work without an administrator password" bit is kind of a buzz-kill.
