Public Domain Information
Copyright protection was always meant to be for a limited time, after which materials would become available to the public for just about any use imaginable. The staff at Cornell University have developed a chart to indicate when works in the United States pass into the Public Domain.
For an interesting and thoughtful discussion of the Public Domain, go to this collection of papers from the Duke Law School Conference on the Public Domain as published in Law and Contemporary Problems at http://www.law.duke.edu/journals/lcp/.
For a graphic look at how many works might have gone into the Public Domain if certain laws weren't passed by Congress, see http://cabinetmagazine.org/issues/10/publicdomainpostcard.php
To determine if a copyright was renewed try this University of Pennsylvania list of copyright renewals, or more information about the process.
In order to get a variety of opinion and topics relating to the Public Domain you can search the archives of the listserv for CNI-Copyright.