In copyright law (at least the American iteration of it, since there is no such thing as an international copyright law), when a work goes into the public domain, then it is supposed to be freely usable. There is a goodly long period of protection for such works, so it is not a case of someone publishing a book today and having it pass into the public domain in 10 years (unless, of course, you are Google, and get the decide what the law means to suit yourself...)
But there is a challenge to that concept, which the Supreme Court has agreed to hear this fall. Here's the story, courtesy of the Chronicle of Higher education's Wired Campus blog:
Library Rights Are at Stake in New Supreme Court Copyright Case
This is not a "The Sky Is Falling" kind of issue for most people. But if you are a library who is trying to make old scholarship available from sources outside of America, a decision to restore copyrights to foreign items that have been in the public domain in the U.S. could have very serious implications for libraries, performers, and the general public.
A good quote from the petition for cert: "“If Congress is free to restore material from the public domain at will,” the petition for review argued, “then the public’s federal right to copy and to use public domain material this Court has recognized may evaporate at any time.”