Oh brother, its hard to believe that people actually buy into conspiracy theories... but unfortunatly, evidence isnt the issue. The more evidence you present, the more evidence the con-theorists have that its ACTUALLY a conspiracy. "Why do we have moon rocks if we never went to the moon?" "Because scientists created those in a lab to trick you about us going to the moon!" No matter what you present, there is ALWAYS an excuse. And then to make matters worse, LACK of evidence for THEIR position seems to make it solid. "Why isnt there any evidence of a 9-11 conspiracy?" "Because it was really well planned out, and people are scared for their lives if they speak out!"
The arguments are always the same, whether its from the 9-11 denier, the holocaust revisionists, the creationists, the moon landing deniers, the einstein (general relativity) deniers, the alternative medicine people and on and on and on.
However, at the root of it all, evidence isnt the issue. The main thing is that people are shaken up that something like the 9-11 attacks could actually happen here. Its more comforting to think that there is something in control (the big bad government) than to think that we are actually very vulnerable and very much out of control.
Here is a Penn & Teller episode about con-theories (has crude language).
http://video.google.com/videoplay?docid=3143048862360929736&q=penn+and+teller