Google just added a new entry into their robots.txt file -- "Disallow: /trends/music?". Well, that would naturally get me wondering what they are trying to hide. It turns out they have a new labs project -- Google Music Trends.
Where does the data come from? Remember the new feature in Google Talk that lets users display what they are listening to in their status message? The new labs project proudly displays the text: "See what Google Talk users are listening to".
There is also a link that says "Participate in Music Trends" that doesn't have any content yet -- but I'm guessing it will take you to a Google Talk download page eventually.
Google just released a new version of the testers version of Google Talk which explicitly asks your permission to share information with this new labs project. See screenshot below.