Whoa! This would be awsome.

A new technology could let your computer recommend new music you might like based on an acoustic analysis of the tunes it already knows you enjoy.
By analysing the characteristics of a song – like timbre, rhythm, tempo and chord changes – then comparing it to a database of a million songs, the software can recommend similar pieces of music, and even rank them by characteristics, like their key or dance-ability.
“The goal is to be able to have as complete a description of a piece of music as a human being can do,” says Xavier Serra, director of the Music Technology Group at the Universitat Pompeu Fabra in Barcelona, Spain. “That’s beyond the current state of our knowledge. But what we can do now is enough to develop music recommendation systems based partly or totally on the automatic generation of descriptors.”

The project called Semantic Interaction with Music Audio Contents has a presentation of its technology online, and is licensing it to private companies.
SIMAC was able to train on a database of a million songs from Sony. For its music recommender, it combines acoustic analysis with a data mining engine that crawls the web looking for music reviews and other textual information that might also give clues to what the piece of music is like.

link- new scientist