Apple's Success Encourages Malware Developers
In the world of malware this more or less means that it's not worth a developer's time to attack anything but Windows operating systems because of the research/development time versus the potential installed base. Now that Mac OS X is picking up popularity, it seems that it is finally a blip on some radars.
“In some cases, attackers will seek to exploit vulnerabilities such as currently unpatched flaws in Apple's QuickTime multimedia player application. In other cases, malware writers will use threats based more on social engineering, such as with the MacSweeper rogue cleanup tool that appeared during Macworld Expo, the researcher said.”