Tobi & Uchiha Madara
First off. Tobi is not Uchiha Madara, or at least his original self.
Secondly. He is not Obito. if you look (here) the left of his face looks unusually well for being crushed by a boulder. His age also does not match up well with anything (Madara, attacking the fourth, etc).
#1. He is too old. Even Orochimaru couldn't defeat age. In the Naruto universe people can live longer than a full life, however it must be by using some type of jutsu. Look at Sasori if you want to see strange jutsu.
#2. His mask. If you look at the old info about Madara he never wore a mask so it is odd he wears one today to hide his face, even when everyone thinks its Madara. It would serve no purpose and only hinder his combat ability. So for some reason he is claiming (or allowing others to claim) he is Madara, however he will not reveal himself. Even after having his mask shattered he WILL NOT reveal his true face, it seems he doesn't want people to be able to look for extended periods. There was a chapter where Kisame was revealing what Madara looked like (here) and it didn't seem as though there were any indications he had his mask on like usual, its rare to show Madara from the back only and he had long hair (which tobi never has). It was so important that Kisame killed himself just to keep what his face looked like a secret. Why would you keep the face of Uchiha Madara secret when people still know what he looks like (also pictures)? Perhaps he had some markings on his face that would clearly differentiate himself from tobi or Madara, whatever it was Kisame was determined not to let them get a modern glimpse of this "Madara".
#3. If you look here he looks very well for being close to 100 years old. Not even the sage of six could defeat age. He also has no battle scars which is strange for having one of the most epic battles in shinobi history. He also looks 30~50 in Naruto universe age.
#4. I have noticed that throughout time Madara acts differently. (here) During this fight now only does he look very young, he also acts different than the other Uchiha Madara's. When the 4th asks if he is Madara he does not respond and only says that "I wouldn't be too sure Madara is dead". However (here) (here) and most current manga chapters he goes out of his way to gloat and tell people he is Uchiha Madara. I couldn't find the chapter page, but one of the chapters he refers to his powers as "my powers, Uchiha Madara's powers". It is very odd that anyone would talk first then third person about themselves.
So who is Tobi?
#1. Uchiha Madara survived and taught another Uchiha his plans and ways then passed down his powers. That Uchiha... etc
#2. Uchiha Madara surivied, got angry, and started a new Uchiha clan. He kept it in secret and taught the members strange new jutsu (somewhat strange since they only have 1 real move).
#3. Uchiha Madara has been doing the Orochimaru thing. If all those Uchiha's he killed he has been harvesting for parts/bodies. If you look (here) -> (here) Tobi lost an arm and got it back. So he could be doing the Orochimaru thing, however that seems somewhat unoriginal for Kishimoto. However, he did have quite a large collection of dead Uchiha so its definately not out of the question. However his technique must be different than Orochimaru since he cannot almost cannot use any jutsu besides some eye techniques.
#4. Uchiha Madara transferred his spirit to someone (uchiha) and is resting in the background until he wishes to come forward and speak a great many things. This is definitely a possibility considering the tailed beasts. Itachi also gave Sasuke Susanoo (its strange tobi doesn't have him) and susanoo has a sword of sealing. Why would Itachi go to those lengths? Perhaps to rip the real Madara out of someone.
#5. Uchiha Madara made clones of himself. Would make sense why he is obsessed with saying he is Uchiha Madara to everyone, while referring to his own power in 3rd person.
I may have left some stuff out, but this is a long post and its late :D I cannot wait to see which method is really used.