For those reading this in other countries, the following applies only
to the United States.

Around 1969 - someone on here will have the exact year - there was talk
of doing a revision of the copyright law. As a result of which, congress
passed a 2 year extension to anything about to become public domain.
When after 2 years they were unable to come up with a bill, congress did
another 2 year extension, and so on, which continued until the 1978
Copyright act was passed, and that law explicitly extended the terms of
all works including those which were extended by prior stop-gap
legislation.

In the provisions below, "any work" means a work for which copyright is
claimed:

1. Any work which was 58 years old before the first extension law -
which was around 1969 or so - is in the public domain as of 58 years from
publication. Those works are not copyrighted any more, and may be freely
used. The only exception to this is the book "Life and Health with Key
to the Scriptures" by Mary Baker Eddy, whose copyright was reinstated by
special act of Congress, the only work ever to be given such special
privelege.

2. Any work published before 1955 whose copyright was not renewed, is in
the public domain. The most famous example of this is the motion picture,
"It's a Wonderful Life" which is almost impossible to find a TV station
running it during the Christmas Season.

3. Any work published prior to the 1978 copyright act which is less than
28 years old, must be renewed after the 27th year or it lapses into the
public domain.

4. Any work published before 1978, whose copyright was renewed and was
not in the public domain before the first stop-gap law, is protected for
75 years from first publication; but it may provide alternately for 100
years from creation, whichever is less. This one I'm not sure about.

5. Any work published after 1978 are protected for the life of the
author plus 50 years, or 75 years if it's pseudononymous or a corporate
author, or a work for hire, or 100 years from the date of creation,
whichever is less.

Any work which is protected under the 1978 law all have their copyrights
expire on December 31 of the year in question.


And there is a nice info directly from the NATIONAL LEGISLATION - USA:
http://www.sice.oas.org/int_prop/nat...sa/clrt17h.asp