UCC: Hyperlinking the world’s books

Last year Kevin Kelly wrote a long New York Times Magazine article about Google Books and other massive-scale digitization projects. The Google Books project, for example, is working on scanning over 10 million university (and New York Public) library books in just a few years. One of the main sites is the University of Michigan Digitization Project, at which Google is working on scanning all 7 million volumes.
In the middle of his article, Kelly writes about the opportunities for user-contributed content to operate on these “universal library” digital collections:

In recent years, hundreds of thousands of enthusiastic amateurs have written and cross-referenced an entire online encyclopedia called Wikipedia. Buoyed by this success, many nerds believe that a billion readers can reliably weave together the pages of old books, one hyperlink at a time. Those with a passion for a special subject, obscure author or favorite book will, over time, link up its important parts. Multiply that simple generous act by millions of readers, and the universal library can be integrated in full, by fans for fans.

In addition to a link, which explicitly connects one word or sentence or book to another, readers will also be able to add tags, a recent innovation on the Web but already a popular one.

When books are digitized, reading becomes a community activity. Bookmarks can be shared with fellow readers. Marginalia can be broadcast. Bibliographies swapped. You might get an alert that your friend Carl has annotated a favorite book of yours. A moment later, his links are yours. In a curious way, the universal library becomes one very, very, very large single text: the world’s only book.

To build successful user-contributed content projects start on top of digitized book collections, we must attend to the incentive-issues, hopefully learning lessons from first-generation UCC projects. For example, as soon as one can annotate, how long will it be before someone starts annotating books with ads offering to sell college students pre-written term papers for sale related to that book? And of course, even sooner we will see Viagra ads.
What about the trustworthiness of annotations? What motivations are provided to encourage people to write (good) annotations at all, and why should they share these with others rather than keep a private collection of marginal notes?