Why do so many of you "rational" people get soooo upset when someone mentions Tobi might be Obito? There's no reason it couldn't be, except for you don't want it to be. And for you conclusion jumpers, before you cream yourself, I do not believe Tobi is Obito I believe as I have for awhile now that Tobi is someone who's name and backstory will be introduced at the same time his face is. I also don't rule out that it could be a past character who's story has not been developed as of yet.
Is it possible? Sure. We've already seen Tsunade say that the name Madara is more important than the actual person of Madara. This is a hint that maybe Tobi isn't who he says he is and could even be different people at different times. Obviously, behind the mask, we have no idea if he's the same person or if he's multiple people working towards a common, long-term goal. At one point, the person behind the mask might have been the real Madara, then handed the identity off to an avatar or a series of them. We've seen also that Tobi typically has short hair but in at least one flashback the masked man had long hair like Madara's. If we go by this clue alone, it suggests that Madara has work the mask in the past and the Tobi person wears it now.
We run into a few problems with this idea, however. We know that Tobi has a pretty unique eye-based space/time jutsu. We also know a few other facts:
1) Edo Madara has displayed many jutsu, but not the same space/time jutsu as Tobi uses, so it's possible that all members of the "Masked Man" persona, if there are multiple of them, do not have it.
2) Tobi fought Minato AFTER the third ninja world war. Obito had already died and Kakashi was still a young man. Obito would not have been old enough or tall enough to be the masked man during this fight, even if he took over the mask later.
3) Tobi has the same general appearance today (same short hair, same stature, sharingan) as he had in the battle against Minato. Tobi today also has the same, unique space/time eye jutsu today that he had back then.
4) Obito could not have been the masked man in the past when he fought Minato. The masked man today is apparently the same one that fought Minato. Therefore, Obito is not the masked man.
5) Obito had only one sharingan, because Kakashi took the other one. However when the mask was broken in the fight against Konan, we clearly saw that Tobi had two sharingan.
6) Kakashi saw Obito die.
7) Yes, a Rinnegan user like Madara or Nagato could have revived Obito. However, there were other Uchiha in the third world war that would have died, and there's no reason given so far why Obito alone, of all dead Uchiha, is one that would be specially resurrected and turned into the current Masked Man persona.
8) There's no indication from the flashbacks we've seen that Obito had the same philosophy as the masked man has with regards to war and peace.
9) Half of Obito's face was crushed by the rock. When his mask was off our view was obscured a little bit but it did not appear that half of Tobi's face was damaged.
Some ideas in support:
1) It is possible that the second masked man (Madara is the first, Obito could be the third) learned the unique space/time eye jutsu and taught it to Obito. This way Obito would know how to do it, but Edo Madara would not.
2) It's possible that Obito has changed his hair to look like Tobi before. He could have changed his entire appearance with a transformation jutsu, but then there would be no reason to wear a mask because it wouldn't look like Obito anyway. We know some people like Tsunade can stay in the transformation jutsu for long periods of time. Again, if Obito changed his appearance he wouldn't need a mask because he wouldn't look like himself (and taking the mask off might reveal a different face anyway).
3) We did see Tobi had a large collection of spare sharingans in his lab. The second eye that he used for Izanagi in the fight against Konan might have been a transplant, the same way Danzo used transplanted sharingans for that technique.
There is some evidence that Obito might be the masked man. However, the evidence that he IS NOT the masked man is much more comprehensive.
Sorry Obito lovers, but right now the hypothesis that Tobi is Obito is just not supported.