I'm double jointed, too and i also can bend only the top joint. I heard it doesn't help playing the piano, but it's indeed good for ballet dancing (well that certainly means double jointed all over, not only on the hands =)
now... how about bending the second joint w/o bending the top joint? can do this finger by finger or all together =) I heard pianists could all do this, but not sure at all... My left hand could do it both finger by finger or all together, but my right hand could only do it seperately...