Digitally of course.
Amazon Media Room- News Release
It would be interesting if you could overlay others annotations while you read the book, which is not possible at the moment, even though the service retains your own annotations.
It could be used for subtle marketing too. Almost like geocaching: The author places constructed annotations in books that give clues about sites or places to visit. What with e.g. the Da Vinci Code containing links to Google Earth for the places visited in the book, etc.