Apple has issued Security Update 2011-003 for OS X v10.6.7 to add the OSX.MacDefender.A definition to Snow Leopard's File Quarantine feature, and to "...search for and remove known variants of the MacDefender malware."

[Edited to add:] The update also will cause OS X to check the Apple mothership for updates to the malware definitions every day.

It does not specifically mention Leopard, the 10.5 generation of OS X which is still technically being supported. Leopard has a more generic warning available that the user has downloaded a program file which could be harmful.

This capability in both Leopard and Snow Leopard is only invoked in file quarantine-aware applications that download files from the Internet, or receive files from external sources (such as email attachments). According to Apple, these applications "...will attach file quarantine attributes. When you open a potentially unsafe file in Finder, Spotlight, or from the Dock, the file quarantine feature will warn you about unsafe file types." So it's possible that if you use another program, such as Firefox, you will not be warned.

But the major problem with this update is that it is already out of date. The malware's authors have already released a new variant that is more deadly. If Apple plans to keep issuing such updates they have a busy summer ahead.