Before people cheer too much, surprisingly works can retroactively be snatched back from the public domain and go back into copyrighted state. The prime example is the movie Metropolis . It became public domain in 1953 but copyright was restored in 1996 only to re-enter the public domain a year ago today.
That being said, copyright is a product of laws and laws can always change. Your comment made me believe that the copyright owner somehow extended their rights; not a literal act of Congress.