I dunno, I remember one Mariah Carey where she sings super high, actually, on one singing lesson website, they described as a "whistle" voice, and it does sound like whistling, it's so high.

Anyways, GuitarTricks has a link to that singing lesson site on the main page. It seems to be pretty reputable. I think I'll start a new thread to get opinions, since I've been considering purchasing the CDs.