Summaries of research reports that suggest that absolute pitch ability can be learned and improved through practice (note that "perfect pitch" is usually called by researchers "absolute pitch"):
Lola L. Cuddy
"Two methods for training the absolute judgement of pitch, reference training and series training were studied. Reference training concentrated during training on the identification of three reference tones in a set of nine pure tones, while series training gave equal weight during training to the identification of all nine tones. Results of pre- and posttraining tests, scored for the number of correct judgements, showed that reference training was more effective than series training for listeners with musical experience. . . . [R]eference training was particularly effective for training listeners with musical experience when the nine tones of a set were grouped into three pitch classes-high, medium, and low pitch."
"Training the Absolute Identification of Pitch," by Lola L. Cuddy, in Perception and Psychophysics, 1970, Vol. 8 (5A), 265-269.
EarTest is a Windows program that allows you to practice perfect pitch (and relative pitch) ear training. EarTest is flexible--you can try all the practice methods suggested by these studies.