Absolute Pitch research, ear training and more
For the "nurture" and learning perspective, Daniel Levitin is a good start.
For the "nature" and genetics perspective, try beginning at the University of California at San Francisco.
Robert Zatorre works with Daniel Levitin and is interested in the neuropsychology of absolute pitch.
Diana Deutsch is the foremost authority on music cognition. Her site includes a study of tonal languages and information about aural illusions.
Robert Goldstone runs the "Percepts and Concepts Lab" at the University of Indiana. Their goal is to explore how human beings perceive their sensory experiences. I'm particularly interested in their publication on perceptual learning, and I expect to figure out how to apply these concepts to learning absolute pitch.
Ernst Terhardt has been working with this subject for decades, and has posted relevant articles about pitch and musical perception.
Kenichi Miyazaki is pursuing experiments on the musical comprehension and performance of people with absolute pitch. His research abstracts are in English, but unfortunately, his full publications are in Japanese.
The Howard Hughes Medical Institute has published an incredible on-line report about the biomechanical processes of our sense organs. I'm amazed because it covers sophisticated, detailed topics yet remains as easy to read and accessible as an eighth-grade science textbook.
If you want to see diagrams of the auditory and vestibular systems, there's loads of em right here, including a simplified diagram of the brain's tonotopic pitch mapping.
This site is for a music school run by a woman who has published her own methods of teaching absolute pitch to children.
Win Wenger, a musician with perfect pitch, has founded Project Renaissance. Win's professional philosophy is that people are capable of far more than they typically accomplish; his website is very loosely organized, but he has interesting theories about increasing your powers of creativity and intelligence.
Daniel Simpson has begun his own exploration of perfect pitch. He has a touchy-feely approach to the material, which contrasts reasonably well with my pragmatic approach.
Kirk Whipple has created a "myth of perfect pitch" page. He has perfect pitch himself, and shares his observations. I have found no research which supports his described method for learning perfect pitch.
The Ball-Stick-Bird reading system asserts and addresses this question: how can you read if you can't remember the sounds of the phonemes?
I'm intrigued by the Golden Ears ear-training course, which is neither "relative pitch" nor "perfect pitch" but ear training for audio engineers.
Wendy at the Center for Musical Antiquities has some interesting items you may be interested in looking at, or buying, including some of Evelyn Fletcher-Copp's work.