When I first flipped through Genii this month the photo of Derek Dingle's hand in On the Slant caught my attention. It probably stood out for me because my knuckles do exactly the same thing. Mr. Racherbaumer dubbed it "hyper mobility". I've always just called it "bein' double-jointed". The only other magician I've noticed this on is Diamond Jim Tyler. He exploits it to do his robotic hand gag.
Out of curiousity - are there any Forum users who possess this "gift"? If so, has it been a help or a hindrance? For me, it's never been a help, but it makes my hands tired as heck when I practice faros.