SynaesthesiaFuture Nobel Prize winner Brian Pilnick mentioned via e-mail that Wikipedia has an excellent entry for synaesthesia and you can read it here--it's very thorough and very interesting.
Here's an additional note from Brian:
I had been reading a little bit about this since I found out recently one of my good friends is color synesthetic with any sound. I've talked with him about it and its really incredible. You can go up to him and ask him what color your voice is. Also, since he can 'see' as well as hear your voice, he can more easily tell changes in your tone so he is very good at telling when people he knows are lying or in a mood etc. My voice apparently turns more blue when I talk to my girlfriend. He describes the color as appearing just outside his peripheral vision, as if he "could see it if he turned his head fast enough." It's all very interesting and music is a completely different experience for him than the rest of us.