<p>I can't figure out a use case for being able to annotate interfaces in Java. </p>

<p>Maybe someone could give me an example?</p>

More an example, but
Local
and
Remote
annotations in EJB3. According to the java doc,
When used on an interface, designates
  that interface as a local business
  interface.
I guess the use case here is that the interface has a special function best denoted by an annotation.