How did September 11th define America? Hotline On Call has a great post going through how Americans view 9/11.
One of the more amazing things the polls find? September 11th is considered as a more defining event than Pearl Harbor:
Whether an attack happens again, in the near future or generations from now, it is no exaggeration to say that Americans look at the world in a different way than they did five years and one day ago. A large number of Americans think about the attacks on a regular basis, are concerned with the future and describe themselves as personally changed or affected by the events of the day five years ago. It may take another generation for those numbers to drop.
No question at all as to whether or not September 11th has defined America, despite what politicians and pundits may argue.