Well, here is the Wikipedia entry on that:
Copyright protection attaches to a work as soon as it is fixed in a tangible medium, whether the copyright holder desires this protection or not. Before the Copyright Act of 1976 an artist could abandon or forfeit their copyright by neglecting to comply with the relevant formalities. Difficulty arises when one tries to apply the doctrine of abandonment to present-day concerns regarding the abandonment or gifting of a digitized work to the public domain. The abandonment of a work is difficult to prove in court, though Learned Hand stated proposed a test which parallels other forms of abandonment law wherein an author or copyrightholder could abandon their work if they intend to abandon it and commit an overt act to make public that intention.[3]