The promised software update to automatically find and remove known variants of the Mac Defender malware has just been released by Apple and it's now available in the Software Update panel or Downloads website. The KB article HT4657 explains Apple has added a "OSX.MacDefender.A" definition to the malware check within File Quarantine. On Mac OS X 10.6.7, the installation process of the security update "will search for and remove known variants of the MacDefender malware". Users will also be notified after a MacDefender variant is removed, and Apple offers more details and information in this article as well.
Mac OS X malware list is now updated daily in the background without the need of a manual software update:
Apple maintains a list of known malicious software that is used during the safe download check to determine if a file contains malicious software. The list is stored locally, and with Security Update 2011-003 is updated daily by a background process.
Security Update 2011-003 provides additional protection by checking for the MacDefender malware and its known variants. If MacDefender malware is found, the system will quit this malware, delete any persistent files, and correct any modifications made to configuration or login files. After MacDefender is identified and removed, the message below will be displayed the next time an administrator account logs in.
Mac OS X 10.6.8 was rumored to be the software update to include a fix for Mac Defender, but it's likely that Apple also pushed a security update for users that will keep running the older 10.6.7 Snow Leopard version, with 10.6.8 getting the Mac Defender fix built-in.
Update: Ed Bott at ZDNet posted two videos showing the difference between Mac Defender's Mac Guard variant in action before the security update, and the new File Quarantine successfully identifying it and removing it after the update.