tobi is madara, even though he is dead, there is a lot we don't know yet and i'm guessing that he was able to split his soul into two or something... or he cud hav been there, like alive at that time, coz wen he removed his mask to show sasuke his sharingan, he had cheek wrinkles, maybe that shows how old he is or something....
The fighting style between the two is entirely different. If Madara had the ability to be intangible, he wouldn't have used the Sharingan on the big guy nor would he have revealed his Rinnegan. I think Madara was alive (maybe not well), and mentoring an upcoming replacement who had promised to give his rinnegan to an Uzamaki upon his death; in order to resurrect him which didn't pan out as well as planned.
I've gone over many things in my head and there is a flaw in every one of them. However, there is one person that I'm currently working on being Tobi and so far I haven't found anything that would go against it (since we didn't know that much about him to begin with): Kagami Uchiha. For him to have been amongst Tobirama Senju, Hiruzen Sarutobi, Koharu Utatane, Homura Mitokado, Danzō Shimura, and Torifu Akimichi, would mean he would be a considerable force to be reckoned with. For one, he's old enough to know all the things he knows now. Two, he was among a Senju and could have gotten his DNA instead of Hasirama in order to use the Izanagi. But, even then, Hashirama's cells were already in great supply on the market. We also have no clue what his abilities are; and, for an elder like himself to be killed by itachi easily would be a feat.
Looking at this images4.wikia.nocookie.net/__cb20110119100415/naruto/images/1/1c/Konoha_fights_fox.png , we can spot the only person unaccounted for, Kagami. "Tobi" in the mean time, was busy with the fourth, at the time of the image.
We know that Tobi can't be the same person as Madara not just 'cause Madara was summoned but also because Kabutomaru suggested "his" motives might not be the same as Madara/he might deviate from the plan, or something along those lines (also, saying "Fake Madara"). I've also taken into account that it could be someone who isn't even Uchiha. For all we know, he hasn't even used Mangekyo sharingan, so no susano'o, tsukiyomi or amaterasu. He also only seems to fight using the intangible and teleportation techniques. And, why use Izanagi (against Konan) when you could use Susano'o?! That said, he could be limited for some reason (my thinking, him not being a real Uchiha). My biggest bet nonetheless still lies on Izuna being Tobi.
