Over at Slate, Paul Collins makes the reasonable point that lots of examples of plagiarism may well come to light as more old works are digitised for Google Book Search. I should hope so! What he doesn't point out is that previous generations of artists have not had the ludicrously restrictive views of copyright and originality that the giant mercenary copyright holders are forcing on us today.
All our most creative people have always picked up stuff from everywhere -- including giants such as Shakespeare, Herman Melville, Ezra Pound and James Joyce -- and made it their own. As Pablo Picasso said: "Bad artists copy. Great artists steal."
If creative people had, for the past 2000 years, been obliged to work under the sort of insane conditions that are increasingly being applied today, we wouldn't have very much culture at all.
I doubt if this is news to anyone, but if it is, try Free Culture: How Big Media Uses Technology and the Law to Lock Down Culture and Control Creativity (2004) by Stanford law professor Lawrence Lessig. You can download it free from http://www.free-culture.cc/freecontent/
