Security research firm Intego, which maintains a Mac security blog that monitors various OS X-specific malware, first noticed and reported the development.  The firm was running the new version of OS X, when they noticed it detected and removed malware.  The process was carried out via a popup window, which they took a screenshot of, but they were either unable to determine or chose not announce who made the antivirus software.

Intego's post indicated that they were not making the product.  ClamAV -- currently the AV engine in Apple's server operating system -- also seems unlikely as the virus detected had the signature "OSX.RSPlug.A", a signature that ClamAV currently doesn't support (ClamAV does have a signature for "OSX.RSPlug" [1]).  Similar, McAfee and Sophos use the names OSX/Puper.a [2] and OSX/RSPlug-A [3], respectively.

That leaves Symantec [4] as one possibility.  Another is that Apple has developed its own proprietary antivirus software -- which would not be surprising.

Assuming that Intego's report is accurate (which seems likely as they're a serious name in the security software industry), it looks like Apple will finally be taking malware on its consumer products seriously.  It should be interesting to see how the program stacks up to the free offering that Microsoft is releasing later this year for Windows 7, Windows XP, and Windows Vista.

For many years Macs remained largely free of malware, while their PC brethren struggled.  This was due to many factors – including a small marketshare and the OS's generally sound design. Additionally, the web-based attacks of today were somewhat less frequent back then because browsers featured less rich content to exploit. 

However, like any OS, OS X was not without its holes -- on both the OS and the application level.  Recently, with more marketshare and Apple's increasing marketing bravado, interest has picked up in attacking the OS.  Recently, a worm attacking Macs emerged, but it appeared to be amateurish, unable to reproduce due to the server it communicates with being dead.  Nonetheless, it seems a matter of time before more serious attacks, implementing the proof-of-concept OS X attacks that security researchers have been demonstrating, come to light.  One such recent proof-of-concept attack demonstrated an an OS X keylogger though Apple has since patched the route it used.