I remember seeing those on someone's road bike and thinking they were cool, I think they said they'd bought them in spain. I've never seen any for sale anywhere. I've come to think they'd come handy in racing, to keep an eye on those cheeky sprint moves behind you, to check if you haven't unknowingly opened a gap, or to see how many people are right behind you.
Has anyone seen any of these? They are tiny (about same diameter as handlebars, and replace a bar-end plug) and fish-eye like to give a wide-angle view.