I'm sure you've seen information on Operation Northwoods... a "False Flag" plan to use simulated Cuban terrorists to initiate an invasion of Cuba. This was confirmed as a suggestion the Join Chiefs presented to Kennedy as a way to being war with Cuba. There are striking similarities with the events of 9/11...
Granted. There are some "striking" superficial similarities between Northwoods and 9/11, which can be an interesting jumping off point for fictional movie or the beginning of a civil discussion about government.
BUT, the critical points with regards to conspiracy theories include;
1) In Northwoods, there were to be NO REAL VICTIMS, and it was rejected anyway.
2) The events of 9/11 pointed the way to invading AFGHANISTAN.
3) And it was "overkill" in every sense of the word. Even a failed "false flag" attack would have been plenty of reason for military action. (Imagine if Flight 93 was the only hijacked plane)
4) The Bush administration _did_ essentially invent false reasons for invading Iraq.
5) And flubbed it so badly that they were found out rather easily without MIB going around "silencing" anyone.
To reiterate then; Northwoods is an interesting place to begin an investigation or a piece of ficiton. It is also a fascinating place to begin a civil discussion about government and morality, but it is neither a conspiracy or evidence of one.
ETA: Ok, you beat me to the post.
ETA: Here's another interesting discussion (not for this forum though). Is it the military's responsibility to at least consider "false flag" operations to advance military objectives? If so, discuss the founding fathers' foresight to make a civilian, elected public official the Commander in Chief. If not, why not?