Jamie Boyle and Duke U's Center for the Study of the Public Domain have produced two excellent reports for the Copyright Office on the plight of "orphan works" whose rightsholder cannot be located. The reports talk about the real-world problems and suggest practical solutions to them.
"Orphan Works" probably comprise the majority of the record of 20th century culture. These works are still presumably under copyright (only works published before 1923 are conclusively in the public domain), but the copyright owner cannot be found. The default response of archivists, libraries, film restorers, artists, scholars, educators, publishers, and others is to drop copyrighted work unless it is clearly in the public domain. As a result, orphan works are not used in new creative efforts or made available to the public due to uncertainty over their copyright status, even when there is no longer anyone claiming copyright ownership, or the owner no longer has any objection to such use.Link (Thanks, Jamie!)