I remember I've seen a little boy on TV who can name the notes his teacher plays on the piano with out looking at her... I guess that's perfect pitch.

I haven't gone that far though, but I can tune my guitar without a tuner almost perfectly... I memorize some of the most played notes very hard (like the standard E & A), so that I can recall them from my memory and hear them in my head, and tune my guitar to it..

I think people are born with this, I believe that most of the people who plays music tend to have it to some extent... but it should be developed by listening a lot of music and playing them.

Also training yourself by doing some by ear transcribtions is very helpfull... someday you'll find your self guessing what notes are being played easier, and figuring out whole songs much faster.