I have a friend who has a website called "Repentance with Joy." I think it captures what repentance is intended to be. If repentance is turning to God, what could be more joyful than that? And that's not to say that repentance doesn't also and first involve a deep regret for one's sins.
I understand people's strong reactions to talk of the need for repentance if they think that people are saying that God caused the attacks. That is to blaspheme God, in my opinion. But if you see that God has simply withdrawn, at least temporarily, the protection that He's had on our nation because we have continuously gone our way (see Mad Dawg's post #22 above), then it doesn't need to impugn God's character.
The good that can come from this tragedy is indeed, repentence and a turning to God. An event like this makes people realize that all their eartthly security is of no use in the face of a destructive force. But God did not CAUSE this to happen, nor did he withdraw some sort of special protection. If you belive we had special protection, then where was God's protection at Pearl Harbor, or the San Francisco earthquake, or the flu epidemic of 1918?
To go farther back, had God withdrawn his protection from the Jews during the Holocaust? How about Europe during the Black Plague?
You see, we can draw INDIVIDUAL lessons from events like this. I can say to myself, "I have been living my life poorly, and want to repent and dedicate my life to Jesus." But as a group, when the same thing is said, then we assume an attitude that is a bit arrogant. To say, "Well, God won't protect us if we as a nation don't repent," leaves a big question in the public's mind. WHO is to repent? WHAT should they repent for? Do you not see that most of those who are saying the nation should repent are thinking "Those OTHER guys better repent?"
I assume that you think that the nation should repent for abortion and sexual immorality. SOME people should. I will also tell you that there are others who think that the nation should repent from racism, or greed, or militarism, or whatever cause happens to be their definition of the worst sin.
We live in a flawed world, in which Satan from time to time attacks with tremendous force. This is what we are living through now.
We are called to humble ourselves, call His name, and seek His face. Whether our nation will be saved or not is not given to us to know at this time. What we DO know is that God keep's His promises, and that nothing, NOTHING will separate us from the love of God through Jesus.