As part of today's Mac OS X Security Update, Apple has added a new signature to the built-in malware checker in Snow Leopard.
Description: The OSX.OpinionSpy definition has been added to the malware check within File Quarantine.
Since Apple introduced this feature in Snow Leopard (we discussed this back in September 2009), they have only updated it twice. The current signature they are adding, for a spyware called OpinionSpy, is something that Intego discovered in June 2010.
This is the first time that Apple is announcing such a change. When the feature was first released, it included signatures for two Trojan horses. Apple later update the definitions to include the HellRTS backdoor, that Intego discovered in April 2010. But this time, Apple is putting its cards on the table, and clearly stating that such a change has been made.
In any case, Intego VirusBarrier X6 protects against all known Mac malware, and its malware definitions are updated at least twice a week, and more often when needed.