I saw this ZDNet article about what could be the first Mac botnet. It looks like the malicious code comes down when pirated copies of iWork ‘09 and Photoshop CS4. No real surprise there. All in all I am not terribly surprised about using Macs as zombies either, I always thought it would happen one day.
With all the talk back and forth about which OS is safer you could find an article or statement to backup any OS. My thinking on that matter has always been a bit more straight forward. They all have vulnerabilities, either known or unknown (yet). If an OS or application (like IE) garners a large enough install base then it would become a target. Simple as that. As more and more Macs become used in the real world I would suppose that more and more hacks, viruses and bugs will become apparent.