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.
In a nutshell, it sounds like the data is compiled from Google Talk users who choose to share such information. What is the number one song as of this posting? 'Map of the Problematic' by Muse.