There are some works that had entered the public domain in the U.S. that have had their copyrights restored (see
Golan v Holder), thanks to the Sonny Bono Copyright Extension Act (CTEA). The Act was enacted to protect works published abroad. One of the results of the CTEA is that no new works will enter the public domain until 2019.
And there are some works which may be in the public domain in the U.S. but are still covered by copyright in other countries.
And a public domain work can have original additions made to it that can have these original additions copyrighted, so you must be sure that the public domain work you wish to use is the original work and not a later version or an adaptation (revision, translation, illustrated edition) which may be copyrighted.
And then you have the odd case like the one out of the Ninth Circuit that held that "Bambi" is still protected by copyright (see
Twin Books v Walt Disney), because the copyright was not secured in the U.S. until 1926.
In addition, there can be trademark laws and privacy laws and publicity laws and unfair competition laws that may come into play when using a public domain work.
It is generally the case that works that were published prior to 1923 are in the public domain in the U.S. and free for anyone to use. But it is not always safe to rely on this publication date and it is always wise to check to make sure there are no other rights that may protect a work, by either consulting with an IP attorney or checking with a copyright clearance professional.
For more information on public domain material dates in the U.S., you can visit the following site:
http://copyright.cornell.edu/resources/publicdomain.cfm or the U.S. Copyright Office website at
http://www.copyright.gov.