i am pretty sure that once a work falls into the pd, it stays there, however if it is still under copyright and the law is changed, the copyright is extended (otherwise, disney wouldn't keep lobbying for longer copyright terms). this latter case would seem like a case of ex post facto to me (shouldn't the copyright in effect
at the time of publication apply ??) but like pshrynk i'm not a lawyer...

that's also why there are some works which are in the pd in the US but nowhere else, particularly the ones published before 1924 (? 1923 ? whatever year it was), or the ones whose copyright was not renewed, at a time when it was necessary to do so (no longer the case).