I can't remember any mention of a medallion in the most recent book. Although the images on the adult cover are always objects that play improtant roles in that book. How did Harry get the initials R.A.B in the last book? Were they on a medallion? Perhaps the medallion has something to do with the final Horcrux?
Most fans assume they refer to Regulus (something) Black, a previously-mentioned relative of Sirius'. It's the best theory I've heard.