I think that may have been the one day (in most recent history) when the world literally just stopped. It was the only thing anyone talked about, all forms of travel in North America stopped, there were police with huge guns everywhere. My mom went to San Fransisco a month after, and she said literally there were Marines with AK-47's every 10 feet. And they had a shoot to kill order, similar to the one they had in the UK during the bombings.
Security in the airports in the US is tighter than Fort Knox. I remember going through the Houston airport, and seriously being scared. We had just come from South America and they were all over us. I'd rather have that though, than have it happen again.