I have gone over the Naruto franchise time and again, and their is no possible way that Tobi is Madara.
It all starts with his appearance, first the characteristics.
   Madara sported long hair, an Eternal Magekyo Sharingan, and serious disposition.
   Tobi has short spiked hair, no siting of an Eternal Magekyo Sharingan (or a Magekyo Sharingan for that matter as far as I remember) and a quite childish disposition.
   So their is no way Tobi is in anyway Madara. Also, he wears a mask that covers one of his eyes, why is that?
   I came to the conclusion that Tobi is in fact Kakashi's old friend Obito, here is my proof,

• Obito and Tobi have the same hair style.
• Obito was known to be slightly more childish, just like Tobi was before revealing his serious nature.
• Tobi's mask is covering the same eye that Obito gave to Kakashi.
• Madara was said to be killed by the First Hokage when trying to attack the village with the Nine-Tails the first time(I don't know who it was the second time, but my guess is that it was an old Madara or a Teenage Tobi that had somehow survived and learned some powerful Uchiha secrets)
• When Kakashi noticed Tobi's Sharingan, he hesitated as if it reminded him of someone.
  
• How Kakashi got his Sharingan and Obito's "death" was what ended Part 1 of Naruto Manga, so it's a perfect spin off.
  
• The recent manga issue of Naruto (#559 Reinforcements Arrive) Kabuto used the Resurrection summoning Jutsu to bring the real Madara into the battle (that was the other coffin in a previous issue that we never saw, the one that Tobi was scared of)
• Also, Obito had a thing for wearing goggles, and Tobi wears a mask.
  

There you have it, all my proof, everything I have gathered the past month or so. If you don't get it, you'll have to fill in the rest of the holes yourself.

You're missing something from your theory- "Madara", or Tobi, fought with Minato 16 years earlier during the attack on the village. Obito, along with Kakashi were still kids around that time and most likely weren't much older when Minato died than they were in the little spin-off manga chapters.

Also, Tobi mentioned how he gave the Rinnegan to Nagato, whom was a kid when he received it. And I may be wrong, but I think that may have been the era before Jiraiya had Minato as a pupil considering him, Tsunade and Orochimaru were still on a team together, during the 3rd ninja war. Minato may have been much younger then, if he had even born. Assuming that's the case, then Obito certainly would have years to go before he was born. So unless Obito survived that fateful day, was secretly alive and grew up the became capable of time travel in a way that has yet to be shown to us and caused those events back in time, I doubt it's really him.

Yes, the whole mask and sharingan thing is a very good supporting idea for Obito being Tobi but that may only be a coincidence. The fact that Tobi and Obito wore something on their head doesn't really give any supporting evidence either. Nor does the hair provide proof. Madara could easily just have cut it that short for whatever reason had he really been alive this whole time. "80 years", as was the timeline from "Madara's" disappearance to Tobi, is a very long time for lot's of change to happen about a person, so you really can't use that as an argument.Having on goggles or a mask is just something to make someone stand out from other characters in the series. Also Tobi's childishness was an alternate personality for him to fool people from knowing his true identity as Akatsuki's leader, namely the guys in Akatsuki, when they were still alive. Not even Deidara knew who he really was he was the leader and he was Tobi's partner. Unless Deidara was the only one who didn't know the true, as Tobi told Kisame that he was last to know he was really the leader. Kakashi's hesitation may simply have simply been him being shocked to see that someone other with the sharingan other than himself and Sasuke was alive. If it's as you say it was, then wouldn't Tobi have been at least slightly surprised to see his old teammate after all this time? Tobi never acknowledged him in a particular way and instead made his debut to them unhindered.

The only other thing that would link Obito and Tobi to be the same person are the letters in their names. But that's the English language. I doubt there are any similarities in the Japanese characters for their names.

Cyro Xero : I know that. That's what I meant by the second one. He was dressed like Tobi, that is why it may or may not have been Madara. That's probably why Minato Namikaze didn't kill him. Like Kakashi, he may have recognized him, and rather than kill him, seal the Nine-tails in Naruto. We'll all find out in a few weeks.

I'm positive that tobi is not madara, because madara was brought back to life by kabuto reanimation jutsu, as for tobi being obito I am not sure its sounds logical, but I haven't done much research on it. 

I've been thinking the same thing op. Especially after today's chapter (#596: One Jutsu) where Kakashi figures out Fake Madara's jutsu. It's probably his eye creating a worm hole much like the portal gun in Portal.

They're insinuating that Tobi is in fact Obito. However, I don't believe that Tobi is Obito. I think it's more likely that Tobi is Madara's brother Izuna. Obito would have been a little kid when Tobi fought with Minato, and so would have Itachi's best friend Shisui. Plus, it never stated that Izuna died. Based on the information given, I can only speculate that Tobi is Madara's brother.

I do not believe that Tobi is Madara. I also do not believe it is Obito. What if Madara used a jutsu to keep him alive like Orochimaru's jutsu? Madara probably found the body and took it to his hideout and implanted Obito's Sharingan in his right eye since Kakashi had the other eye.