The main thing that has always bothered me was the inside trading. Put options were made on the United Airlines, (a sort of bet that the stock will drop) just before the attack. And not just a small amount either, it was many times the average. http://www.hereinreality.com/insidertrading.html

Also, while I don't like to speculate what could have happened at the pentagon, it just seem ridiculous that an aircraft could enter the airspace of the pentagon without them knowing it. No anti-air missles were fired from the well known batteries sorrounding the pentagon, and Mineta testified to this.
Why on earth would Cheney purposefully order the batteries down, unless he wanted the plane/missle/whatever to hit the pentagon?

Also, I think the poster asked what the signifigance of building 7 was. Well, if explosives were planted in that building, who is to say they weren't planted in all of the World Trade Center buildings? Not only that, but Larry Sylverstine knew about them.

Also, Jhon O'neill quit his job at the FBI because of repeated stonewalls by Bush himself to stop investigating Al' Qeada and Bin Laden. Very suspicious stuff sorrounding that.

Now, I won't go so far as to say that the Bush administration 'planned' the attack, but I certainly wouldn't put it past them to let it happen.