Despite all the angst on the fifth anniversary of the event, and contrary to popular belief, the 9/11 attack and the collapse of the World Trade Center towers did not transform the design of tall buildings. It's still business as usual in the world of skyscraper design and construction. And that's the way it should be, for 9/11 was a security failure, not a building performance failure.
The lesson from 9/11 is not that buildings should be designed to withstand attack by airplanes; it is that criminals should be prevented from flying airplanes into buildings. The nation has responded appropriately to this lesson, by tightening security to the point that an airliner deliberately being flown into a building cannot reasonably be considered a "design event" for the building.