That was a disingenuous argument at its best, because it always was vulnerable - just less so. Now, as the Macintosh goes more mainstream and increases in popularity, people who get their jollies by writing malware are targeting Macintoshes more and more.
The current preferred method of attack seems to be through Java apps on websites that access the Mac OS through the installed Java application on the Mac that runs the webapp. Last week a large attack titled "Flashback" infected anywhere between 700K and 800K Macintoshes. Apple was slow with addressing this, but finally came out with a security update last week. Mac users need to take a cue from their Wintel cousins, and be a little more cautious about what and where they are clicking out there in the wild web world.
More information can be found here, and here. Tools have been released to address the potential vulnerability if you need to check. Security companies had tools out before Apple had the fix to the OS addressed, so if you are prone to Java play on the internet with your Mac, you may want to check this out.
To suffer a flashback of a different type and quote an old 1980's police TV serial, "Let's be careful out there."