Sometimes you have to read between the lines.  In one of those innocuous stories that AOL and CNN love to pick up, Malcolm Gladwell, William P. Young, & Dan Brown Top "Most Highlighted Passages of All Time" List on Amazon Kindle, there is the wonderful quote:

"AmazonKindle also introduces a "Popular Highlights" feature that identifies the passages that are most highlighted by the millions of Kindle customers. We combine the highlights of all Kindle customers and identify the passages with the most highlights. The resulting Popular Highlights help readers to focus on passages that are meaningful to the greatest number of people."

Of course, as a few people have noted on Twitter, this means that Amazon can, and does, track every bit of text you underline.  Never thought of that when you decided to buy the Kindle version rather than the printed book, did you?  Who gets to see what you think is worthy of underlining, and what other uses are made of the information?