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:

http://naruto.wikia.com/wiki/Setsuna_Uchiha