Guest blogging for Larry Lessig, Judge Posner offers an interesting idea:
Lawrence Lessig: Bill Patry, a distinguished copyright lawyer and treatise writer, and I have written an article soon to be published in the California Law Review in which we advocate an interpretation of 'fair use' that would solve the major problem that extending the copyright term creates. We argue that it should be considered fair use to copy an old work if the copyright owner hasn't taken reasonable steps to provide notice of his continued rights, as by entering his name and address in a copyright registry. Given such a rule, such registries (which have counterparts in the case of works of visual art) would spring up overnight. Then if an Eldred wanted to publish some old work, he would consult the registry or registries and if no owner was listed (which would usually be the case, because most old works have no commercial value and so their owners won't bother to try to keep them from falling into the public domain), he could publish it without a license.
I think this underestimates the extent to which conglomerates will protect their backlist, while not keeping it in print, but it's a start.