I think Bruce's answer is spot on in explaining what the fallout from an atrocity like the Boston Bombings is like:
1) People catching flights around the country will be incovenienced by the TSA,.
2) Law enforcement, the government and the FBI will easily be able to lobby for more powers.
3) Fear, mistrust and hatred of Muslims (and foreigners in general) will increase.
I do wonder who a situaton like that actually benefits though.
By the sounds of it, it's neither the people trying to promote Sharia Law, nor the immigrants intent on settling and contributing in the US peacefully...
In response to Bruce's answer though, I feel there are several points worth adding:
People in the countries you're talking about hate terrorists more than you do.
Imagine having a "Boston Bombing" every single day of the year - that's pretty much what people in Iraq have to deal with for example. I can't imagine the terrorists who kill innocent civilians in these countries get much loving.
I also think its naive to believe many migrants want sharia law. In most cases people leave strict Islamic countries like Iran because they DON'T agree with it.
The ones that are "hell-bent on spreading Sharia law" abroad are the equivalent of dumb hicks and are luckily few and far between.
For some reason though, they seem to get a lot of media attention - kind of like filming a documentary in some backwater town in the Deep South and broadcasting it to the world saying this is what the entire US population is like (no disrespect to the Deep South - I like chicken and biscuits just as much as the next fella).
Also worth mentioning that the Islamic country with the strictest Sharia law is Saudi Arabia.
If the US shut its doors to them it'd be saying goodbye to the 52% net imports of petroleum from Saudi controlled OPEC countries.
Also, according to the Washington Press (m.washingtontimes.com/news/2006/jan/15/20060115-103622-3038r) Saudis had up to $800 billion invested in the US economy as of 2006.
That has likely increased dramatically in the past few years.
So unless you want to see a big jump in unemployment and people cycling everywhere - I'd suggest being nice to at least some of the immigrants.
Final point - you mentioned "Islam and its radical regimes".
I honestly don't think religion has much to do with murderous regimes.
For example, the Syrian government promotes secular politics (where Religion is separate from government).
The US actually backed them until they killed 70,000+ of their own people in the last 2 years.
I get where you're coming from though - the whole point of terrorism is to create animosity and fear, which is what a lot of the people in the world are feeling right now..
In these scenarios, shutting the doors seems like a sensible thing to do.
I just think everyone needs to learn to get along, and perhaps start questioning the world around them rather than taking everything we hear on Fox News/"from some bearded cleric" at face value.