This is very interesting to me. A while back I was fascinated with the concept of absolute pitch (some people have a qualitatively different, categorical perception of pitch) and did quite a bit of research on the topic. One resource that I invite anyone that is interested to read is the research done by this guy:

http://aruffo.com/eartraining/

Ignore the fact that he has a "learn AP here!" link - usually that is a signal that it's a scam of some kind, but in this case he has quite a bit of interesting material to back his theories if you go to "AP Research".