In comments to my previous post, Patty of Oboeinsight asks if absolute pitch can be taught. Do a google search for Learn absolute pitch and you will find many claims that you can do just that. The French conservatory method of ear training uses fixed-Do solfège in efforts to develop absolute pitch. My conclusions are based upon my own development of quasi-AP and from listening to countless undergraduate and graduate applicants to the Eastman School of Music as an Aural Skills supervisor.
First, adults without AP can develop a memory for specific pitches. I was required to memorize A440 as an undergraduate student, which I was able to do. I can still reproduce this note whenever I want, though my accuracy is not exact (± a quarter-tone). I can also reproduce a G when I think of the beginning to Mussorgsky's Pictures at an Exhibition, or a C# for the beginning of Mahler's Fifth Symphony. This is because I practiced these trumpet excerpts over and over for many years, so the memory was reinforced to the point of permanence.
Second, this ability will deteriorate very quickly if it isn't practiced constantly. Students who learned AP through fixed-Do solfège did not do well at sight singing, especially if they had finished their solfège training a year or more before the audition. My ability to produce an A fluctuates with how much I practice it, though lately I've been continuously reinforced from my daughter's Suzuki practice.
Third, some people still show more propensity to develop AP skills than others, suggesting a possible genetic component.
Finally, the quasi-AP that is learned is not the same type of pitch perception as "native" APers. A person with true AP places any pitch automatically within its proper pitch-class category. AP-as-a-second-perception does not engender this automatic labelling.
For some articles on Absolute Pitch, start with W. Dixon Ward's chapter in The Psychology of Music (Diana Deutsch, ed.) Next, Daniel Levitin's "Absolute representation in auditory memory," Perception and Psychophysics, 56,(1994), 414-423; R.W. Lundin, "Can perfect pitch be learned?" Music Education Journal, 69,(1963), 49-51; and K. Miyazaki, "Absolute pitch as an inability," Music Perception, 11,(1993), 55-72. Heck, just look through many of the articles in Scholar Google. Fortunately, much of the research was done under the auspices of NIH, so it is available online.