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Today, Apple sent an email to Mac Developers, who do not put their apps on the Mac App Store, and encouraged them to get ready for Gatekeeper:

Gatekeeper is a new feature in OS X Mountain Lion that helps protect users from downloading and installing malicious software. Signing your applications, plug-ins, and installer packages with a Developer ID certificate lets Gatekeeper verify that they are not known malware and have not been tampered with. Mac Developer Program members can sign applications with their Developer ID now to get ready for Gatekeeper.

The theory is that when Mountain Lion is released with the default setting to only install apps signed by Apple, many novice users will keep that setting and thereby only install apps from developers that Apple signed-off on.

However, as we have seen in the iOS App Store, developers can be malicious, steal data, or worse in rare occasions. In these instances, Apple would be able to shut off access to those applications from that developer.

Apple sent a similar email two months ago.