Security Update 2011-003 protects against Mac Defender and future malware.
Apple released the promised security update that protects OS X users from getting infected by MacDefender and future malware. The solution checks daily for updates to the File Quarantine malware definition list. The update searches for and deletes MacDefender if found on installation.
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Right now there are no other malware definitions that the security update is evaluating. You could opt-out of the daily check, but I think it's wise to have Apple provide you with malware updates for the future. Apple decided to act to keep OS X safe for the most part. There will be more security issues in the future as OS X adoption grows though. Apple needs to act a bit faster next time.
You can trigger the Security Update 2011-003 update download by hitting Software Update right now on your Mac. See also this support note on the Apple site.