The Intego blogs talks about "reports we've seen" which state Snow Leopard comes with anti-virus/malware functionality built-in, and also provides a screenshot of how it looks. This dialog appeared after downloading a file through Safari, and the operating system detected the RSPlug trojan.
There's no way at this point to confirm this news, but it's interesting nonetheless. Only today did Apple release two Get a Mac ads which played on the rather outdated mantra of "thousands of viruses" for Windows, so if Snow Leopard came with built-in anti-virus/malware, that is sort-of funny.
Joshua Long theorised which anti-virus/malware scanner Apple chose to include with Snow Leopard; ClamAV was a logical contender, but as it turns out, ClamAV's engine labels the above trojan differently. Intego and Symantec do label it as such, and of those, Intego obviously couldn't be it. Could this mean Apple licensed the engine from Symantec? Or did they develop their own?
It is important to stress that there is currently very little to be worried about when it comes to the Mac and security. While several security researchers claim Mac OS X is easy to hack, large-scale infections have yet to take place. Still, proper security policies should always be enforced.