A new favorite podcast is Future Tense from American Public Media. Last month they reported on Pandora, "an Internet music service that streams songs based on 400 distinct musical characteristics."
The people behind Pandora are working on a music genome project, so if you say you like Al Stewart, they'll play music for you which "features major key tonality, demanding instrumental part writing" or "mellow rock instrumentation, folk influenes, mild rythmic syncopation, melodic songwriting, and acoustic rhythm piano." It's $36 per year, but you can listen to 10 hours for free.
The Genome Project, according to the Pandora site "capture[s] all of the little details that give each recording its magical sound - melody, harmony, instrumentation, rhythm, vocals, lyrics ... and more - close to 400 attributes!"
Anyway, check out Pandora and the Future Tense podcast (RA req'd)(other topics include Google stuff, the DMCA & listening to electronic music, and using the Internet to help victims of Hurricane Katrina).