I've wondered the same thing. So far, the only thing that I can find that will fit in the clip is my ipod mini. But even that is a little too thin, and falls out of the clip from time to time.
I'm sure at some point, somebody in the aftermarket will find a way to attach something to the clips, or make them useful somehow. I can't think of anything myself, but I wouldn't be surprised if there will be something to stick in there....