I can't help but point out that this scenario has already played out, but for the Iraqis. America's actions have resulted in the deaths of far more than 30,000 civilians. Now you may consider those deaths justified, collateral damage if you will, but I can sure as hell say with confidence that those on the recieving end will see it as the indifference to human life not your kin that it is. What is the proper Iraqi response to this? For the more extreme of you, who buy into the tripe that Islam is the true enemy trying to turn you into their untermensch, would those 30,000 deaths that, were they American, would have you drooling for them, any them, to be 'turned into glass', are those 30,000 deaths justification for the scenario offered by the OP? Eye for an eye? Now, I don't believe that, but you have to admit you believe the reverse, or that the 3K deaths on 9/11 are all the justification needed for the more than tenfold response in lives in Iraq alone.
I'd say that a nuclear detonation in an American city is as an unlikely situation as you'll get, but I'll play along. I'd like to add, that this, the worst case of what a terrorist can do against us, nuking a city, is a far cry from the absolute death of humanity that was faced during the cold war. Keep that in mind, that it's not the end of the world, and only your resulting actions could turn it into that.
If I'm not mistaken Cheney is already on record as stating that if America were attacked again on the scale on 9/11, they'd launch an attack against Iran, even if they were not behind that attack. If you truly understand the implications of a war against Iran, then pity America, pity her that she would give a blank check to those in power in order to seek retribution, as she did after 9/11. See, thats what would happen, the event would be exploited by those who could gain politically by it, to accomplish their goals and objectives, whether or not that would better america; it'd better their cause.
The slide towards authoritarianism in America, that seems to me has let up a little as more and more see how bankrupt bush and his personality cult are, would increase, without reservations or regard to checks on power and accountability. Those who now fester in anger that their glorious adventure in Iraq has thrown up a roadblock in their ambitions, and tarnished America's good name, nature and intentions, would be renewed in their collective hate towards the "ism" that keeps them in the pocket of the status quo and those who benefit by it. They were right all along, and the liberals who opposed them were in fact traitors, and should be hung. Their dubious schemes, perhaps rounding up all the arab muslims, or torturing the living daylights out of any captives of that same demographic, would be given green lights.
Those that define themselves through war, and you have to admit, there is no other nation today that does so as much as the united states, would have further cause to trumpet the expansion of militirism, to buy that extra F-22 on credit.
I'd say that sure, there would be a large economic impact, even more so if an Iran attack led to the cutting off of the Straight if Hormuz, but 300 million Americans have no other boat to turn to, they'll keep living their lives, struggling along, even as a portion of what they send off to Uncle Sam is directed to the murder of others, and less than 1 percent of them actually have to do the dirty work. They'll work toward keeping the gears and sprockets turning, even as the conniving few take control of their collective effort and manipulate the american economic powerhouse and war machine to their own ends.
As to the idea that usually plays out in these sorts of discussions, that Mecca and Medina should be counterattacked, well, thats just the most asinine idea ever. Seriously. Sure, I'd imagine that there would be calls to do so from the usual suspects, and perhaps they'd be heeded as those in power seem to have that same rabid thinking, but that would truly be a piece of nutball manifest destiny, believing so much in the false ideology of 'clash of the civilizations' confrontation that you go right out and create it. There are over a billion muslims; if the Islamic fascism theory were true, then there would be a far greater amount of 'terrorism' then there actually is; you'd be knee deep in carnage. A billion muslims, yet less than a thousand suicide bombings since the 80s. Terrorism is overblown, and human ingenuity is the reason 9/11 succeeded beyond success and blew your mind, not that there are hordes of enemies waiting to strike at what you hold dear. However, the eradication of the the holy sites in Islam would turn that crazy belief into reality, with a billion people now your enemies until the end of time, although perhaps that is what those who await the rapture truly want. It's what Osama truly wants, an insane overreaction by America against the muslim world, so that the muslims would rise up against their apostate states and overthrow them, and the only reason that might make the OP's scenario credible, although OBL getting his hands on enriched uranium or plutonium and the bomb to detonate it is about as far out unlikely as you can get.
I guess, Frackal, what I see out of your thought experiment, is exactly what occurred post 9/11, only writ large, and a with a much much more dark vision of the future, as the sheer horror would grant the far right in your country to free themselves from any restraint they might have, and allow them to silence their appeasing liberal traitor critics. One nation under god would be the outcome, I'm sure. It's unlikely and I don't fear it, but que sera sera.