The title is incorrect: OS X 10.8 (Mountain Lion) by default can only run signed apps. All appstore apps are signed apps, as are anyone who gets a free signing

The title is incorrect: OS X 10.8 (Mountain Lion) by default can only run signed apps. All appstore apps are signed apps, as are anyone who gets a free signing cert from Apple. There is a setting to "only install Appstore Apps" but it is not the default.