since the release of 559 everone's starting to think that tobi is Izuna, but there were other characters that followed Madara's ideals (the original Madara) one such character being Uchiha Setsuna . So I think that Setsuna may be Tobi who attacked Konoha with the Nine tail he even had many of the features we've seen of tobi. read more about him in the link below:
This is my guess, the guy behind the mask is Uchiha Izuna. And you know how the uchiha clan has to take someone else's eyes to reach the eye's full potential? I think that Madara took Izuna's left eye, and Izuna took Madara's right eye. If you notice when Madara shows up in ch 559, only his left eye is shown. It's possible he doesn't have a right eye there, or at least has some other kind of eye.
But that's just my guess. Man, Naruto is getting very exciting. I can't wait for the next chapter.