The situation I find interesting is not necessarily 'orphan' works, where you cannot find the author, but a book that is in copyright but out of print without sufficient demand to reprint it (something similar happens with CDs). For example, I do business with someone in Asia that was very interested when I described the Smithsonian Collection of Classic Jazz CDs, which are well within the copyright period, but have been cut from the catalog.
Copying the CDs for him violates the copyright, but the only other way to legally get this series is to find it used. Somehow copyright law should take into consider availability as a criteria ie, as long as the work is available new or has been in the last x years the copyright is valid and enforceable, otherwise the work falls into the public domain. The 'x' could be, for example, 10 years, or 50% of the initial copyright period. After all, the copyright protects intellectual property but is also in part to protect the profit of the copyright holder, but if the work is out of print that 'protection' is somewhat nebulous at best.