The probability that Tobi/Madara is lying at the moment is essentially 0 given our current information. Why? We can look at it both from a plot and plot-design perspective.
Why give us a whole, giant, multi-chapter LIE? That would only make sense if it was super dramatic and could have rediculous ramifications later. The "lies" Madara are telling now would only increase the gap between them, as Sasuke isn't about to join his clan's murderer. If he had lied and said he wasn't involved, THEN he could have had a chance at Sasuke.
Think about it. What is the ONLY other reason given for why Itachi killed his family? "To measure my container." That translates to, roughly, "To see if I could."
Well, he admitted to Sasuke that Madara HELPED HIM. Know what that makes his previous statement? A LIE. If he was being helped, he wasn't identifying his capabilities.
This is the only other explanation being offered for why Itachi did it, and several chapters are being dedicated to it.
Not to mention that it never made sense (until now) why Itachi stopped fighting his cousins and father that one day when Sasuke pleaded with him. Or why he brotherly poked his brother's forehead right before dying. Or how he went to talk with Naruto? How about when he cried after hearing that Sasuke died? Wouldn't you think, if he cared so much about Sasuke's eyes, that he would have gone to fight Sasuke right away instead of letting Deidara go and risk them being injured in the fight?
Sasuke's overall strength wouldn't have mattered if all Itachi wanted were the eyes. Factor in how he gave Sasuke his techniques and designed a protective system for Sasuke using them.
Yeah. Madara is SO lying... *rolls eyes*
And for people pointing out that Sarutobi wouldn't do that? Actually, he probably would have. He was a peace-loving man, but he was also a ninja. He was considered to be one of the most powerful shinobi of all time and was the Hokage during a war with the sand when Kakashi was a boy.
There is something about the Uchiha clan that we don't know; that is painfully obvious. Nothing about them adds up. Why is it, that if Madara (their leader) was such an enemy of Konoha and the First Hokage was their enemy, they became the Konoha police force?
Why did they need so many clan meetings, which Itachi was terrible at attending?
We see when they are fighting Itachi that the 'thugs' in the police force barely even know the law, and call for Itachi's arrest and interogation because someone with a Sharingan could have possibly forged a suicide note. Last I checked, the clan was FILLED with Sharingan users. Not to mention that they randomly don't know that they cannot arrest an ANBU.
Itachi served directly under the Hokage, yet the clan was incredibly angry when ANBU missions kept him from attending their meetings. Shouldn't such a powerful, respected clan know that Itachi's first duty was to Konoha and his mission?
If the Uchihas were involved in the Kyuubi attack that nearly decimated the village and cost the life of a Hokage and cursed his son, I think he might be pissed enough to have them punished. Especially considering that peace could have been impossible if the Uchiha's plans were accomplished.
Sarutobi is a warrior like everyone else. He fights for the greater good. If that meant killing the Uchiha clan, of course he would have done it. And he would have done it silently so that Konoha didn't destabilize worrying about the major clans.
Sarutobi never stopped clans from creating side families or offering them up as sacrifices so that the main branch would live, such as with Neji's father.
He may have been a peace-loving man, but he was a fool. You can see it with how he was with Naruto. Instead of teaching Naruto and personally interacting with him along with the village, he put a taboo on the Kyuubi attack. Had he been more proactive, Naruto probably would have grown up in a much more caring environment.
Ever notice how, when Naruto thinks about the "bonds" he has made, the Third is never first? First comes Iruka-Sensei, then Kakashi-Sensei, then Sasuke/Sakura then the Third with the kids from Konoha.
Had Sarutobi been more responsible, Orochimaru never would have become the power-hungry man he was. As his teacher, Sarutobi could have prevented that.
Don't get me wrong, I love the Third. But he was a fool.