I remember being impressed by this at the time:
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.
If you do not wish to receive these updates, you can disable daily update by unchecking “Automatically update safe downloads list” in the Security pane, in System Preferences. This option appears in Security preferences after Security Update 2011-003 is installed.