Yesterday I also stumbled across
The Music Genome Project?
It supposedly has differnt attributes of songs listed in some massive database, or something. Name a song, it will play other songs 'of a kind'. It's sort of of like the last.fm thing, but without the silly users listing all sorts of odd tags innapropriately. The only danger is that it may have what I'd call 'innapropriate' tags that I can do nothing about. There's a 'I did/n't like this song' option, so that's good.
I guess only experience will tell. If I end up checking the "why did you play this song" button too many times it's no good.
[apparently
Neil Gaiman is a fan. I'm becoming one myself after finding how to get the right combination of artists/songs so I get stuff I'll actually enjoy.]