I don't think it's Zetsu; How would Zetsu have the Sharingan? Then again, who/what Zetsu is is still a mystery. It's true that he's genetically modified from Tobirama, so I suppose it's possible he's a mix of Tobirama and Madara's genes or something.
Either way, it's guaranteed that Tobi's name is a mock of Tobirama's name. I suppose it could be a Zetsu clone (or his real body), as that would explain why he seemingly don't have any of Uchiha's powers other than Izanagi. Still, I think it's a friend of Madara, or Madara in a new body (like Orochimaru). He might have split his soul in two, or something. It's obvious the real Madara knew of the plan, as he was disappointed he wasn't resurrected in the way he wanted to be.
Although it would make sense; Madara seem to have some trust issues, and creating a clone of himself mixed with Tobi's genes would ensure that his plan goes through, I suppose. Or perhaps Zetsu was created to make sure his ally goes through with the plan.