There are some interesting points made by the people that believe this theory.
Take the last words recorded on one of the planes to run into the wtc for example. I see water and buildings ... Oh my God! Oh my God.
Now first of all are you telling me that a stewardess doesn't know when the plane is decending? Second, this stewardess doesn't recognise the wtc on a clear sunny day. These women would recognise Chicago or Vancouver but they didn't recognise NY?
There are some other interesting points however unlikey this scenerio may seem. Unlikely but interesting. Like the fact that the WTC had traded hands just prior to it's destruction with a new insurance policy that included terroriism insurance.
I don't believe it but it's great stuff for the conspiracy theorist.