Over the last few days, heavy users of the Mac App Store have been reporting that applications that have been purchased as recently as November 10 are failing to open, or reinstall. While the commonality of the apps affected has yet to be identified, Tweetbot developer Paul Haddad has determined the problem to be an expired security certificate.
Haddad discovered that the certificate expired on November 11, and the problem did not affect all users, or even a large percentage of applications. Without fanfare, Apple issued a new security certificate overnight, which has rectified the problem, at least in part. Users were able to completely bypass Apple's Gatekeeper to run the applications in most cases.
MacNN testing has shown that the re-issued certificate doesn't always fix already installed applications. In those cases, users must reboot and/or re-download the afflicted app in question for a complete fix.