Apple is publicly acknowledging that its Mac is not virus-free -- at least as much as it's ever going to say it.
Apple apparently changed the wording on its Mac site from "It doesn't get PC viruses" to "It's built to be safe," according to Sophos which released a "Before" and "After" look at the Apple site.
Previously it read, "A Mac isn't susceptible to the thousands of viruses plaguing Windows-based computers. That's thanks to built-in defenses in Mac OS X that keep you safe without any work on your part.". Now that is changed to something much less definitive:
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"Built-in defenses in OS X keep you safe from unknowingly downloading malicious software on your Mac."
Obviously this has something to do with Apple's run-in with the Flashback virus which affected around 650,000 Macs earlier this year. Its very public crisis made people question Apple and it's "virus-free" working on its site.