Conspiracy? No. Intentional negligence? Absolutely. It's a well known fact by now Bush was given security briefings which pointed out in no uncertain terms that Bin Laden posed a serious and imminent threat to the U.S. And yet these briefings went largely ignored.
What I believed happened is that Cheney and those pulling Bush's puppet strings were dead set on getting those military defense contracts, and thought the threat of/attempt of a terrorist attack would play into their hands beautifully. What I don't think they counted on was Bin Laden's ability to actually carry out a plan of such immensity with such a large death toll. The U.S. being the cocky bastards that we are, was lulled into a false sense of security and thought we were untouchable. And when shit got real, the administration capitalized on the attack by hauling ass over there to grab the oil.
So while I never believed they had knowledge of it beforehand, I think they anticipated that Bin Laden would make an attempt but not succeed. Then they could come in with the fear mongering and use that as an excuse to utilize the defense contracts by going after the terrorists. Like the saying goes, once you make people afraid you can make them do anything. Clearly and rightfully people were terrified, and they capitalized on that.
I remember watching a documentary about how the Halliburton truck drivers were earning about $6000/wk just for hauling the oil from field to site. Meanwhile they had even gone so far as to outsource all of the military auxiliary jobs to their own defense companies as well. Instead of having military personnel doing things like laundry for instance, soldiers were paying these contract companies $99 to have their laundry done. Just disgusting.