Syferz;32261789 said:
1. Kabuto calls him Tobi, and then says sorry you go by Madara these days.
2. Kabuto shows Tobi the coffin and tells Tobi that he can't refuse him. Kabuto's 6th coffin holds someone Tobi can't defeat and can't be known to exist.
3. Kabuto reassures Tobi that he hasn't told a soul yet.
4. Kabuto tells Tobi by addressing him as Uchiha Madara: Uchiha Madara was in a class of his own, and Tobi replies "wise guy" meaning that Kabuto isn't paying Tobi a compliment but saying that his deguies was a very logical choice.

This all means that Tobi is not Madara, and Kabuto knows it, and more importantly Tobi knows Kabuto knows it now after seeing the 6th coffin.
Madara Uchiha is the 6th coffin, proven 100%

i edited my OP and gave more detailed arguments plus manga proof that tobi is the real uchiha madara and not the one in the casket.


thesis:
tobi is not really uchiha madara but the 'thing' in the casket rather is (5) the real uchiha madara.

evidence: the vague/mysterious statements of kabuto.

see: http://www.onemanga.com/Naruto/489/17/
kabuto: i felt that first i would come greet you, tobi. excuse me, you go by madara these days, don't you?

and madara's shocked reaction to kabuto's edo tensei and especially the last summon (so called "the ace in kabuto's sleeve"), which sort of blackmailed madara to submit to an offer he can't refuse:
see: http://www.onemanga.com/Naruto/490/03/
http://www.onemanga.com/Naruto/490/04/

anti-thesis:
tobi is the real uchiha madara, and the 'thing' in the casket is something/someone else other than madara.

here are the manga facts:

*Tobi claims that he is uchiha madara,
proves he has the power and knowledge and intentions of the real uchiha madara
,

and these guys attest that he is:

*itachi, add to this sasuke's theory that itachi could not have single-handedly taken out the whole clan, much less the entire police force that has all the best uchihas in the clan, including their father (who is but the biological source of these two geniuses!)

*kakashi,

*jiraiya,

plus *minato's testimony that the 'masked man of akatsuki was the one who summoned and controlled the kyuubi in attacking konoha and is extremely powerful and wise that he has read all he was about to do,
and

*all the rest of the akatsuki accepts he is (including kisame[!] who also called him mizukage),

synthesis:

(1) kabuto's statements are intentionally vague and mysterious at best; and tobi's reactions are even much more vague and mysterious. it is like being in a kick-ass poker game, nothing can be what it seems until the last cards are dealt on the table.

anyone can put an interpretation to these statements and reactions according to his/her own intentions and agenda; but there is a high degree of qualification. these interpretations must take into full consideration the already established facts in the manga and not ignore them altogether or wholesale.

(2) to grant that tobi is NOT madara we should be able to effectively and totally refute the above claims and evidences stated in the manga (which are presented under the anti-thesis);

or at least offer an alternative reading to these testimonies that would offer an equally-convincing or better interpretation to these characters' identification of tobi and madara as but one and the same person.

(3) but if tobi is NOT REALLY uchiha madara
then this tobi guy must really be a good impostor(!?!).


but he should not merely be impostor who claims the same name but one who really knows madara and konoha's history, is more powerful than madara to be able to appropriate or possess his UNIQUE powers and his knowledge. he was able to fool everyone else, perhaps even himself (he really believes he is madara!) -- well, except of course, kabuto (!?!).

so if tobi is NOT REALLY uchiha, who else could he be?

(4) so far we don't have a candidate that would fit the description.

that's why the genius kakashi said:
"i don't have any solid proof, but considering his plan, i doubt it could be anyone else."

of course, add to the 'plan' evidence is tobi's ability to cancel/nullify his kamui
(for sure, kakashi would have pushed through with his jutsu since he has already decided that it is crucial that he takes out madara this early. UNLESS tobi/madara prevented him not only with words but also with a counter-jutsu to an MS, or that he was really immune to a a weaker type of space-time jutsu, the kamui could not work against him.) see: http://www.onemanga.com/Naruto/487/04/


conclusion:

this means that not only does tobi claim the name uchiha madara, he also has the same wicked intentions, historical knowledge, and the power/sharingan to control the kyuubi (and no one else has been known to control that beast, just one, according to jiraiya, no one else but madara) and counter/nullify an MS jutsu.

therefore, it makes total sense that tobi is really uchiha madara.

while the claim that tobi is NOT uchiha madara, but the 'thing' in the casket instead does not make much sense and goes contrary to the evidences and testimonies replete in the manga.