I think that this is probably one of the most disturbing revelations of the novel. Changez's reaction of smiling is a complex one and it is important to not fully dismiss it as "anti- American" or something that reduces its intricacy. Hamid's entire premise is that there has to be an open and thorough dialogue on the issues that define our being and our world in the modern setting. This would be one of those instances where assessment has to be wide ranging and inclusive.
On one hand, it is sincerely disturbing to note that Changez would be smiling at the planes hitting the buildings and the instantaneous nature of death on the morning of September 11. He rationalizes it with the moral equivalency of how the American must have been smiling at the bombing of Iraq. I assess this as disturbing, as well. Changez's "smile" though is not necessarily at the events or the death that results. His characterization has been one that has had to deal with the weight of "difference" in living and working in America. Changez has perceived his reality of one in which he has had to be "American." In the fulfillment of this ideal, he was doing things he would not have done in Pakistan but in living and working in America made them acceptable. We sense that this was a type of dual consciousness for Changez consisting of the American element to his identity and the observer in the back noting how different he actually has become in his pursuit of being American. Whether or not we agree with this or whether this was something that Changez had to do might not as important than what he felt he had to do. Changez felt that he had to embrace this burden of consciousness, of being. With the attacks of September 11, there was an instant where he felt America experienced for a moment what he experiences daily in terms of having to be brought down to one's "knees" for what one is. Again, this can be debated as to whether or not this actually exists, but the reality is that this is what Changez perceives. In this perception, his smile is at such a statement of being. This is part of the characterization that we have seen in Changez, so that his reaction has to be assessed with this in mind. Understanding this, I think that we can find his reaction to be disturbing, but also a reflection of a deeper set of values in reality that have to be examined and reexamined. It is here where I think that Hamid is striving to have Changez studied, for it is not his reaction to the events of September 11 which is as important as the conditions that lay the groundwork for them. In doing this, Hamid might be asking us, the reader, to assess the events of September 11 in the same manner. He might be asking us to consider the implications of events that led up to that horrific moment as opposed to simply dwelling on it in isolation.