well i have to agree with what most of you here think that Tobi is Izuna and Madara is the one who is in the 6th coffin coz that was also my first theory..my second theory is hmm Madara could also be using a technique same as Orochimarus body transfer technique..hmm i dont know what to call it but you know what i mean hehe..since he use that technique it is possible that he really got killed by Hashirama..and about Hashirama, his wife came from Uzumaki clan who are known for their advanced sealing technique..Hashirama could possibly sealed Madara's power before he died which means his power is sealed within his original body..and now Madara is using Tobi as a substitute body with only half of his power..what do you think?