Pandora radio is an interesting project. It's an automated streaming radio system which uses something called the Music Genome Project to guess the listener's musical taste.
Back in 2001, I started subscribing to a similar service called Launch.com, in which you rated music, and a database of what everybody likes attempted to guess your tastes. It was a great idea, and worked about 70% of the time, except it was based on other people's preferences, and I am almost certain that it tipped the scales in favour of owner Yahoo!'s commercial partners.
The Music Genome Project, on the other hand, has actually categorised every single song in its database with categories like tonality, rhythm, time signature, vocalist style and gender, genres (down to the finest distinctions), instrumentation, lyrics, aesthetics, song structure, etc. It got embarassingly good at guessing my tastes really fast. The good thing is, that it continually explores the edges of your tastes, guessing what you'll like and sending that music your way, noticing what things you go for and what things you don't.
Me, I seem to have a preference for meandering phrasing, odd time signatures, jazz influences, electronica influences, minor AND major tonality, acoustic sonority, male and female simple or dynamic vocalists, ambient soundscapes, unique forms, expanded chords, folk influences, etc. How about that?