It was my understanding that US Copyrights were given an extension that kept books that should have entered the public domain from doing so. Anything pre-1923 is public domain but other books wouldn't begin to enter the public domain until 95 years after publication.
See
http://copyright.cornell.edu/resources/publicdomain.cfm
Here's a link on the page you linked to explaining why we don't get any new public domain books in the US
http://www.law.duke.edu/cspd/publicdomainday