In the quest The Red Crystal, you discover what looks as if it's part of a humanoid jawbone, encased in a red crystal, which has fallen down the mountainside from Felwood. This could be part of the Skull of Gul'dan.
That's actually a very good point, and I think I might have part of a possible answer. The orcish skull Illidan is clutching in the BC cinematic is missing its jawbone, so perhaps it is indeed a lost portion of the skull.