[ZGoten]

Maybe Tobi is a resurrected Madara, created by somebody using Edo Tensei. It would explain why he's still alive, and probably why he's merely a shadow of his former self (if there is quality reduction). This way, maybe Kabuto was summoning another Madara, basically saying: "You want to play games? I can play games, too." Tobi's surprised (and slightly shocked) face may be due to the fact that he didn't know there could be multiple resurrections of one and the same person. However, given the fact, that a person is revived by using another body, I don't exclude it from the possible.

The only questions remaining would be who summoned Madara, what happened to that person, and why does this Madara only have one Sharingan (or does he)?