Everyone in Akatsuki should have seen Madara's face, except Itachi. I remember when he showed his face to Kisame, (in real time and not the time during the Kisame flashback where he meets Madara/Mizukage) Madara took off his mask and said 'Sorry Kisame, you're the last one to know, since you were closest to him (itachi)' or something like that. By 'last one' it seems he's talking about last one in Akatsuki.
Deidara knows quite a bit too, since just before the living Deidara died, he looked to the sky and said 'sorry tobi'. Tobi/Madara must have revealed his plans and possibly his face to him too.
Off topic question: I wonder who was the leader of the Uchiha clan just before the massacre. This person must know/hold many secrets of the clan, past and present, and i would expect him to have MS to become leader. Surely, Itachi would have some difficulties taking this guy out.