Gatekeeper not blocking apps? On Mountain Lion, I've got Gatekeeper enabled and set to allow Mac App Store apps only. Now: if I download any app from the internet (and I'm talking about apps I've never run before) it just runs, Gatekeeper seems to be not working at all.
Does anybody know what's happening?
As bogdansrc mentioned, Gatekeeper only applies to application bundles that have a com.apple.quarantine extended attribute. Some applications (like μTorrent) don't add them to the files they download, and some unarchiving programs (like unzip) don't add them to files after extracting archives.

You can list extended attributes with xattr -l.

Gatekeeper is also disabled if you have disabled LSQuarantine with for example defaults write com.apple.LaunchServices LSQuarantine -bool false.