This would never have occurred to me, but then I'm a guy. In the past 48 hours I've heard directly and indirectly from three completely unrelated women that they want a mirror for the back of their iPhone. By this I gather they mean a "pocket mirror" or "purse mirror", not the quarter inch reflective dot on the back of some camera phones and not the synchronization software of the same name.
There's certainly plenty of space, assuming you don't mind covering the Apple logo.
All you have to do is avoid the camera lens in the upper left corner. I'm surprised the 3rd party market hasn't jumped on this.