In the wake of high profile attacks like Flashback, and allegations that they are '10 years behind Microsoft' in terms of security, Apple has decided that it might not be the best idea to say that Macs don't get viruses. With that in mind, Apple changed a description on its website from 'It doesn't get PC viruses,' to 'It's built to be safe.'
Once upon a time, Apple's 'Why you'll love a Mac' page said that Macs weren't susceptible to the thousands of viruses plaguing Windows-based computers, which is technically true. But even though Macs aren't vulnerable to Windows viruses, it's still possible to build viruses and malware for a Mac. Basically, that's like Microsoft pointing out that Windows doesn't get Mac viruses.
You can take a good look at the changes for yourself over at Sophos. While Apple still touts the same security features, they're a little bit more cautious about promising total immunity.
Meanwhile, Apple is working to improve the security of OS X with Mountain Lion, which give users the options of using Gatekeeper, which prevents users from installing non-screened software and encases them in an iOS-style 'walled garden.' Of course, you can always override Gatekeeper, or just disable it entirely.
And to be fair, there's no arguing that there are way fewer viruses for the Mac than there are for Windows. Of course, whether that has anything to do with Apple's security features, or just the Mac's much smaller market share, is open to interpretation. That much is obvious from the splash that Flashback managed to make.