You certainly all have noticed how much Kishimoto enjoys putting hidden hints and weird sentences all around the pages of his manga. It might be a bit frustrating sometimes, but it contributes to making the manga's universe look deep and rich - just like the real world's history is sometimes made of thousands and thousands of synergies of events, and sometimes was forged and led by the iron fist by just a bunch of men.
About when Naruto fought Sasuke in the valley of the end and almost got killed. Can you remember Kakashi and Pakkun looking at the two statues and saying, "it's ironic they fought there" and "their lives are strangely similar"? At that point, I can remember I was like "Who the heck are those two guys and what about that "strangely similar bullshit""?
Well, after taking a closer look we could notice one of those two statues was the 1st Hokage's (as we saw him fighting Sarutobi under oro's jutsu during the attack of konoha) but no more. Who the second statue was - Uchiha Madara - wasn't to be easy to find out before long, and how their story was similar to Naruto's and Sasuke's was even more obscure.
THE FOLLOWING CONTAINS SPOILER
Very little though.
It's actually very easy to understand. Madara and the 1st Hokage - ie. respectively the founders of Uchiha Ichizoku and of Konoha no kuni - have been the closest friends for a time. We now know, thanks to the 370th chapter, that Madara apparently really is the one who consciously released the Kyuubi. Let's see, what was the deal already...
Ah, yes. He released the demon in exchange for a small part of its power. He needed this power to fight his father, the Karasu Tengu, who had already got rid of his mother and was intending to kill him too to use his body as a container for the sake of god-knows-which-dirty-old demon-geezer plan.
Madara, in lust for power, unleashed the Kyuubi and let the evil chakra take over his body. When the 1st learned what he had done, he considered it as a betrayal towards Konoha (which is quite logical) and there they fought in the Valley of the end, causing overwhelming damage to the surrounding areas.
Sasuke and Naruto are pretty much the same, not exactly though. In order to kill his brother, Sasuke drowns into darkness to acquire power. He lets the cursed seal take over him and joins Orochimaru, an embodiment of evil (though, contrarily to Madara, Sasuke doesn't put Konoha in deadly danger doing that... he just joins an old enemy of it, but still this is betreyal/desertion). Naruto, on his side, is desperate by the betrayal of his friend and fights him to "get him back".
That's what the "irony" Pakkun and Kakashi spoke of on that day was all about. Just so those of you who hadn't noticed yet can now notice. ;-)