While Gatekeeper is enabled in the OS X Mountain Lion developer preview to those testing it, it has also been included in the latest Lion release (OS X 10.7.3), although hidden by default. Apple has informed developers that they can choose to enable Gatekeeper for testing purposes in 10.7.3 without having to update to the Mountain Lion developer preview through the command-line tool spctl(8). In order to do just that, you’ll have to open up Terminal and issue the following command: “sudo sptcl –enable”. You can also replace “enable” with “disable” to turn Gatekeeper off once it’s been enabled.
Gatekeeper functions within Lion the same way it does in Mountain Lion, preventing the installation of malware by blocking applications that were not created by “identified developers” or downloaded from the Mac App Store from launching. There is really no reason for the everyday user to enable Gatekeeper in OS X 10.7.3, it would only be useful for developers to test how their apps will behave with Gatekeeper enabled.