Condi Rice said, in May, 2002:

I don't think anybody could have predicted that these people would take an airplane and slam it into the World Trade Center, take another one and slam it into the Pentagon; that they would try to use an airplane as a missile, a hijacked airplane as a missile.

Well, guess what: they were warned, specifically, about terrorists plotting to fly airplanes into buildings:

In 1998, US agencies discovered that "a group of unidentified Arabs planned to fly an explosive-laden plane from a foreign country into the World Trade Center" - but did little about it, the 30-page report [on intelligence failures in Sept. 11th] says.

That's a report from the BBC on an inquiry led by Eleanor Hill into intelligence failures leading up to Sept. 11th -- strangely, google news turns up nothing on the report, and on the inquiry, only a subpoena tiff with the FAA...

Ooops: Forgot to say that I got this via The Agonist bulletin boards...