Tobi's identity.. Lets settle this once and for all
What we know and what we can safely assume.
Tobi knows much about Madara and has a Sharingan with the ability to use it effectively (a la flicker style) and NULLIFY Kakashi's attack. This makes him Uchiha for sure. Interesting enough, he sports a fan like the one Madara has. Surely he also carries a Kama attached to the end of it. This combined with specific weapon and tool training that can be clan specific, I also agree with the previous posters and feel that Tobi is [wait for it]..
He has knowledge of Madara's secret wall. Meaning he has read it before. Since you have to have the Sharingan and Rinnegan to read the entire thing, he possessed them both at one point or he knew Madara personally (keep reading).
Lets not forget that some records show Zetsu is also from Konoha. His relationship with Zetsu, Danzo and his knowledge of when to strike to free the 9 tails (which was kept super secret.. at least they tried to keep it super secret lol) leads me to believe that Tobi is also from Konoha as well.
Tobi claims that he "as Madara" gave the Rinnegan to Nagato. Nagato had these eyes when he was only a child, but he was not able to control the power. These were clearly transplanted into him and he had to learn to use it like Kakashi had to learn to use the real power of his Sharingan. Since he was not an Uchiha, it turned Nagato's hair white and he could not harness the full power of it. Who's eyes are these?
These eyes that Nagato had was in fact MADARA's eyes. We can see that even though the eyes are transplanted, the IWR versions still use the eye technics they developed.
IWR Madara is aware of Nagato and furthermore, aware that Nagato has (or could at least develop) the power to ressurrect someone (He implies that he knows that Nagato learned to harness his power.. the power of the Rinnegan).
Madara knows of Orochimaru and knows that he had the knowledge to use IWR and Kabuto tells him that he is 'his' apprentice. Tobirama created IWR and Oro would have been a child at the time of his death. This would explain why Oro knew about the Jutsu and only had to find out how to use it. WIthout a doubt, Madara is aware of IWR thanks to Tobirama and references it as a complete atrocity against life and death in the universe. Until Muu mentions it, he believes he was summoned by Nagato. Unless Madara was still alive when the story line started (which I doubt), this would mean that he worked with Tobi to plan his resurrection (and told Tobi how to prepare Nagato for this ultimate plan!!).
Tobi stated that Nagato should have been the third SO6P and that he was the Second SO6P. So what do they have in common? SO6P had the power of the eyes as well as the physical body. Nagato being a member of the Uzumaki clan also had these traits and could have been Jinchuuriki for the Ten Tails.
So.. the SO6P was a combination of Body and Eye. He was literally a SAGE (Senjutsu) like Naruto and J-man and he also possessed the advanced eye techs of the Uchiha. Credits on the fact that he has the eyes of a sage when we can actually see his face. We see that IWR Madara now has Wood jutsu and he looks at his body during the last manga. Surely, his body has some Senju enhancements thanks to Kabuto having captured Yamato, as well as, having Zetsu and Hashirama's cells to work with.
Madara was able to develop the Rinnegan AFTER his battle with Hashirama. A battle where he absorbed some of Hashirama's power... This is HOW he was able to develop the Rinnegan and why no one after him could do so.
Right now, Tobi has the Sharingan and the Rinnegan with the majority of the Jinchuuriki. He is able to control the bodies and share lanes of vision with Rinnegan. He is also able to control the Jinchuuriki within them with the Sharingan that have been transplanted. This means that he also had knowledge of the fact that IWR Madara could awaken the Rinnegan.
I think the others are too old to survive all this time, but it makes perfect sense that this person is roughly the same age as Danzo while still being functional on the battlefield.
On an additional note:
We can safely assume that the Rinnegan is needed to control Gedo Mazo. Once Tobi obtains the Rinnegan from Nagato, he is able to summon it. Gedo Mazo has "digits" which could be percieved as "tails". In all pictures there are at least ten of these digits. Even in the pictures of the Jyuubi, you can see additional "digits" coming out of its back (note these are in addition to the tails). Tobi is able to control him in this stone state as Nagato was able to. More credits on the fact that this is the the shell of the Jyuubi that was created after SO6P used Chibaku Tensei.
Finally, why would Madara think that Nagato was able to bring him back? The ULTIMATE power of the Rinnegan is to bring people back to LIFE. NAGATO DID THIS FOR ALL OF KONOHA because a short time had passed. He MAY be able to do this if a long time has passed to bring ONE person (MADARA) back to life.
So we come to the end of this long tale. Who is Tobi?
My vote is for KAGAMI