Finally we have some light:
Basically, your library of tracks is compared to all the other Genius users’ libraries of tracks. Apple then runs a set of previously secret algorithms, which Goldman described as straightforward recommendation algorithms similar to those used by other services like Netflix when it suggests movies for a user to watch now or add to his quene, to generate statistics for each song. “These statistics are computed globally at regular intervals and stored in a cache,” notes Goldman, because data on the similarity of any two songs changes slowly–it’s assumed the only reason it changes at all is because of the changing tastes of the listening public, and the introduction of new tracks and artists.
Goldman jokes that if he told you how Genius works, he’d have to kill you (or at the least, have a squad of police officers raid your brain to retrieve Apple’s rightful property), but he continues to describe how the program works anyway.
via Technology Review: Blogs: Guest Blog: How iTunes Genius Really Works. (via Glenn Peoples @ Billboard)