If you want more detail, this article in Wiki is interesting: http://en.wikipedia.org/wiki/Absolute_pitch

As a trained performer/music teacher, I have found no evidence that 'Absolute/Perfect Pitch' can be taught. 'Relative Pitch', which I mentioned above, CAN be taught & improved upon. It's a slow process, requiring persistent repetition, enormous patience & a clearly defined goal as to exactly why one would need to develop this skill. Abandon your electronic tuners & develop your 'Relative' tuning skills!
                                        Nigel.