Apple updated Java for OS X Lion and Mountain Lion Wednesday with the release of Java for OS X 2012-005. The update tweaks Java controls by automatically turning the Java plugin off when no Java applets have been run for an extended period of time.
Apple's patch notes also specify that if users hadn't installed the previous version of Java (Java for for OS X Lion 2012-004), that the Java plugin will be disabled immediately.
In either case, you can reactivate it by clicking on the "Inactive plugin" region on a webpage with an embedded Java applet that you want to see.
Apple has not yet added this update to its Apple security updates page, meaning we don't know the specifics of security fixes included in the update.