Paul Durrant in a comment on the TeleRead blog cited a very interesting factoid:
In the US, at least, you can’t claim copyright on an out-of-copyright work just through simple mechanical reproduction of the work.
According to Wikipedia:
Bridgeman Art Library v. Corel Corp., 36 F. Supp. 2d 191 (S.D.N.Y. 1999), was a decision by the United States District Court for the Southern District of New York, which ruled that exact photographic copies of public domain images could not be protected by copyright because the copies lack originality. Even if accurate reproductions require a great deal of skill, experience and effort, the key element for copyrightability under U.S. law is that copyrighted material must show sufficient originality.
The full text of the decision can be found online right here.
While the original decision was about photos of artwork, the implication for digital archives is enormous. This decision, if applied to sites like Google Books and the Internet Archive, would simply mean that no copyrights can be claimed in any shape or form over digital copies.