I believe the old man at the end of chapter 601 is Izuna! Madara has been revived to a young state, has the Rinnegan, and the Mangekyou sharingan in both eyes. But the old man in the end of 601 only had a regular sharingan in one eye, and seems a bit crusty! Maybe Izuna didnt die when Madara took his eyes, I think somehow he got another set of eyes and lived on! In chapter 601 madara says "well its you" why would he say that about Obito? He would say that about his brother whos been on his side since the begging... The body we see is definitely Obitios, I dont doubt that, but I think Izuna did the Soul Transfer Jutsu Orochimaru did.. Cuz Oros best techniques are Edo Tensei and the soul transfer jutsu, and we all know Oro does forbidden Jutsu, for a jutsu to be forbidden someone else had to previously do it! SO to sum things Up I think IZUNA did the Soul Transfer Jutsu on Obito 