the real Madara was in the coffin. Tobi is just an impostor, you see if Kabuto showed that Madara is really dead then, Tobi's plan will fall out of place. the question is who is Tobi?
i think Tobi might be Madara's brother "Uchiha Izuna", i think that after Madara died (by the 1st hokage) Izuna took his eye's back from Madara's body, regaining his sight. he wanted revenge for his brothers death so he attacked the leaf using the 9 tails but failed because of the 4th hokage, after he realized that he needed to do something big, so big that no-one will ever be able to stand in his way, the "Moon's Eye" plan. the story never tells what happen to Uchiha Izuna but i think it's posable..... but will just have to wait and see