All of Abigail-II
comments are right on the money. A good resource for this is Brad Templeton's10 Big Myths about copyright explained
Personally, I'm surprised that caching sites such as Google have survived legally unscathed. I find it interesting that Yahoo avoids using this feature in their Google powered results. Perhaps there will be battles on this in the future.
For my own work in hymnology, I spend a lot of time with old publications that were once under copyright but have now gone into the public domain. I am very careful to restrict myself to editions that were published long enough ago so that they have clearly fallen into the public domain everywhere in the world. It is possible to take a public domain work, alter it, and claim copyright for the edited version (this is how critical edition publishers of music such as Henle and Wiener Urtext make their living).