Apparently there may be genetics involved and there appears to be a greater percentage of asians who have perfect pitch.
That may not be due to genetics, but rather due to Chinese being a language where pitch/intonation is critical to meaning. This was mentioned in HH's thread on early music education recently.
Yes, I completely agree with Twinergy! Being a music teacher, I found that perfect pitch be trained. Like Twinergy mentioned, if you have heard and sing (hum) the "sound" for a zillion times, you trained your brain to "memorize" the "sound" of a particular pitch.
In my many years of teaching, over 70% of my students have developed perfect pitch and the rest have excellent relative pitch. This training is done at a very young age through lots and lots of singing.
Glad to have you with us, dopeywong! How do YOU go about teaching perfect pitch? Is it something you teach specifically (ie., devote time to dedicated exercises that are for developing perfect pitch)?