Not sure I bet it's plastic..Shoot em a line, I am curious too.
I just found some more information on the material used for the iFrogz BackBone. It's the same "soft" finish used on their Luxe iPad 1 cases. They describe is as a polycarbonate with a "velvety feel". Seems like that kind of a finish might attract a lot of skin oils. I'll probably take a look at some youtube reviews of those to see what people thought of that material.
http://ifrogz.com/blog/3830/ipad-2-case-sneak-peek/