Comment by _ph_

No, a database with the hash of every program you ever started on that computer. The Apple server should be contacted when a program is installed/run for the first time. And of course, the system software should handle network problems more gracefully. If everything "works" when offline, network problems should quicker lead to the offline behavior. There should be one daemon process which handles the signature checking which changes its behavior once requests to the server are not answered.