The flip side of copyright is the public domain, of course. It's by far the larger field, though you wouldn't know it from the attention we lawyers pay to the rights side of the coin. But because copyright is time-limited, that smaller field of rights is the one that trails most closely behind us, is nearest to us; and so the works under copyright are those we are more aware of and those more closely pertaining to the issues of the day: if copyright originated on our yesterday, the public domain is what we find when we look back to the day before yesterday. But the fact remains that all those days before "yesterday" yield a vast body of works of the human imagination.
Figuring out what's in the public domain can be a challenge, however. This is particularly true in the United States, where, to quote Kenneth Crews, who is the Director of Columbia University's Copyright Advisory Office, "The law of copyright duration is a mess." This has spurred him to publish a paper, ""Researching the Copyright Status of a Book"" [PDF] that takes you through the process of figuring out the duration of copyright on a particular book. Crews points out that this quest has been made a good deal easier because of the Google Books project, which has scanned a number of volumes of the U.S. Copyright Office's Catalog of Copyright Entries (otherwise available only in paper at some libraries), which tell you whether the original copyright has been renewed. (See Searching Google's Scans of the Catalog of Copyright Entries.)
The process of determining whether a work is in the public domain in the U.S. is also available in chart form on the Cornell University website in PDF.
The situation in Canada is somewhat easier. Creative Commons Canada devised a flowchart [PDF] to help you determine whether any work is in the public domain. And there were plans, announced in 2006, for the creation of a registry of works in the public domain through a joint effort between Creative Commons Canada and Access Copyright. However, the project has since been abandoned, alas.
Interesting post and thank you for the links. A further complication arises when trying to determine who owns a work once you find it is under copyright protection. (Particularly difficult for works such as old movies, where the soundtrack and the cinematography may have different owners).
I'm not sure about the public domain being larger than that protected by copyright though. While there are thousands of years of history which are now in the public domain few examples of these works have been preserved, and the human population and literacy rates were significantly lower.
There are probably more 'works' created each year now (hundreds of thousands of books and millions of emails, computer programs etc.) than we have remaining from previous centuries.