Beginning around episode 140 of Naruto Shippuden, after Itachi's death, Tobi is seen going around a lot, either with Sasuke, or showing his "real?" face to Kisame as the one who is controlling the Mizukage.
It seems that the general consensus from what I've heard is that Tobi is Obito, but Kisame says it is Madara when he seems him, and of course Tobi tells Sasuke that he is Madara, and it seems like Itachi believed Madara was alive so it would seem to add up that he was Madara.
Is the Tobi that was doing all of this Obito, disguising himself as Madara, and for what reason did he need to?