The update delivers three new or modified features.
Firstly, installing the update checks for and removes known variants of MacDefender.
Secondly, MacDefender is added to the list of malware detected by the operating system's File Quarantine feature. If MacDefender (or one of the other blacklisted items) is downloaded and opened, the user is warned that it is harmful and should be moved to the Trash.
Finally, daily checks are made for updates to Apple's list of known malware. An opt-out mechanism is provided for this feature.
Since the File Quarantine feature is part of 10.5 and 10.6, the lack of a 10.5.8 version of the latest update suggests Apple has dropped support for Leopard ahead of the release of Mac OS X 10.7 Lion. While the company doers not make public its formal policy on support for previous operating system versions, it's general practice with Mac OS X has been to deliver security updates for the current and previous major versions.
Security Update 2011-003 can be installed through Software Update or may be downloaded via Apple's Support Downloads page.