Most people that have seen Inception are speculating on wether the spinning top fell at the end. In my opinion, it wouldn't matter. Let me explain...
First of all, you'll have to understand how a totem works. A totem is a small item to help you keep track of reality. With a totem you'll be able to tell wether you're in someone else's dream or not. So Dom's totem, the spinning tactricoid top, would fall if he's in reality, but also if he's in his own dream.
We have several examples of totems in the film; Arthur's loaded dice, Eames' poker chip and Ariadne's golden bishop. All the other totems work differently than Dom's. Arthur's dice always shows the same number in reality because one side is heavier than the others. Other people don't know which side is heavier, so basically the dice could show any number in someone else's dream. Ariadne's totem works similarly, since it's also loaded. Also, Eames' poker chip has a word misspelled on it in reality, while when in someone else's dream the word is logically spelt correctly. So when in a dream all totems act like you would expect them to act in reality. The exception is Cobb's totem. Why would all people keep a top spinning in their dreams except Cobb himself? His totem really makes no sense when you think about it.
Also, you are the only one who's supposed to know how your totem works, because otherwise the purpose of the totem is defeated. This is what Arthur told Ariadne: "This is a loaded die. I can't let you touch it. That would defeat the purpose. See, only I can know the balance and the weight of this particular, loaded die. That way when you look at your totem, you know beyond a doubt that you're not in someone else's dream". Well, there are several people who know how Cobb's totem works. For instance, it used to be Mal's totem, so she knows. Ariadne knows as well, because Cobb told her. Who knows who else he has told.
So this means Cobb could be in reality or his own dream, even if the top would topple. There's a small chance he could be in someone else's dream as well. And, this also means we don't really know wether Cobb is in reality during any part of the film. My theory is that the whole movie is a dream of Cobb's. More about that some other time.
And yes, I know about the wedding ring theory, but personally I do not believe the wedding ring is a valid totem. Anyone could dream Cobb with or without a ring. The people who work with him know that he always wears his ring while in a dream, so they naturally dream him with the ring. But if Cobb personally believes he's in reality when dreaming, he wouldn't dream to have the ring on his finger.
So, personally, I would rather speculate about what Cobb's son says at the very end of the movie; "we're building a house on the cliff". Cobb just came out of limbo were Saito had built a house on the cliff. Is this a coincidence, or do you believe Chris Nolan had some deeper purpose to that line? Please, feel free to share your thoughts.
Update: When surfing the internet I found an interesting comment about James' last line in the movie. I quote;
"The movie starts off with a house on a cliff. It ends with a house on a cliff. The limbo sequence with Mal also features that unbelievably familiar house on a cliff. It would seem that houses on cliffs are associated with the dream space, and we see that multiple times throughout the movie. And in any case, why would a kid in real life build something as specific as "a house on a cliff" with toy blocks? May be that's not the most valid point, but definitely something to think about..." (source: here)
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