My view, and it's not for everyone, is that looking for proof of a conspiracy is stupid. If you believe that the conpirators are halfway competent individuals, as they should be, then there will be no proof. If they were doing mind control **** back in the 60s, am I to believe that "they" would be incapable of pulling off a simple hijacking in this day and age?
Of course, there are problems with this mindset. First of all, you very quickly end up questioning everything, including the existance of reality itself, if you pursue the line of thought. Also, people have been proven to be much less competent than they are made out to be in the imaginations of writers and directors, though this could itself be a conspiracy, where certain events have been intentionally revealed to give people a false impression of incompetence on the part of the government. I'd do that if I were in "their" position.
I usually just split the difference and give everyone a little credit. So my positions is that while the US probably didn't orchestrate 9/11, they may have known about it and let it happen. I have no proof either way, but going down the middle is a nice safe bet.