A member of my family is an interesting case, in that he technically doesn't have perfect pitch, but does for all practical purposes. He has a clear memory of a particular note that he heard as a child and can recount that memory. Years later he is able to identify the pitch of anything by comparing it with that memory. He might think about it for a second or hum a note to himself, but invariably can identify the pitch without have to hear a reference (apart from the one in his head).

He's got the best of both worlds, in that he doesn't get driven crazy by something being out of tune, but can accurately name a pitch without a reference. He's also been able to transpose on the fly from an early age.