Think about it. Even though its anonymous, Apple still has loads of data now on what people's libraries are composed of. When you start Genius, it sends your entire library of song information to itunes. All they do is have to datamine all this collective information and they can use it to predict what will sell. The users are collecting the data for Apple. All the work is done for them. Seriously, they should win an award for this idea. It's going to be one of the largest scale music marketing enterprises in existence.