Here is a graphic from a CNET article yesterday stating that more than 600,000 Macs are infected with the Flashback botnet.
According to the story, the malware is designed to steal personal information. It was originally spread by masquerading as a flash installer which required the user to approve with a password to install on their computer. It has evolved over the past months and is now exploiting a Java vulnerability to propagate. Apple has released patches for Java here for OS 10.6 and here for OS 10.7. CNET has an article about the Java issue here as well.
In short... the days of Mac users getting to be blissfully clueless about security on their devices may not yet be over... but they are numbered for sure. My recommendation is to keep your software current, to spend $4.99 and buy the Intego's Virus Barrier Plus via the App store on your computer, and to be smart about what you do on the computer and don't just click yes to allow anything that ask for a password to install on your mac.
This was inevitable as the Mac grows in popularity and acceptance. This also means that, with the meteoric rise of iOS devices in popularity, we can expect more attacks on our iPhones and iPads moving forward.