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"):
Jasba Simpson and David Huron
"As already mentioned, much existing research has already implied that absolute pitch is acquired via learning. The results here support this view, but furthermore imply that the mechanism by which absolute pitch is learned is the same as for innumerable other forms of learning investigated by psychologists."
"Absolute Pitch as a Learned Phenomenon Evidence Consistent with the Hick-Hyman Law" by Jasba Simpson and David Huron, in Music Perception, Winter 1994, Vol. 12, No. 2, 267-70.
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.