Iran could spend the money on purchasing the material on the black market if they really wanted to hit America with a dirty bomb.  They'd save a lot of money and have a lower risk of getting caught. 

I suspect this is very much the case... especially the latter half.

Honestly, I've become skeptical that it is/would be "relatively easy" to create and smuggle such a weapon into a U.S. port, but that may just be ignorance. It would be interesting to read some actual empirical factual information on this instead of just arm-waving. I agree that these seem on the surface of it to be real and serious threats worth considering, but if that is true, why have no such strikes been carried out in the last 6.875 years when Al Qaeda escalated the Islamofascist War on the West, and the U.S. went into semi-offensive mode? Since 2001, probably hundreds of thousands of Muslims have died at the hand of Coalition (meaning Western forces). The situation in Iraq has gone from victory, to quagmire, to better, to worse, to seemingly (in the minds of most Western media) hopeless, to now "unimportant" by virtue of the fact that there is not much "bad news" to be told. In short, "We are winning," and the Islamofascists are "losing." You'd think that during all that time, if attacks like those you outline were real prospects that Iran or whomever would have carried it out, woudn't you? That makes me skeptical that any claims that such a threat is a major one might just be speculative if not alarmist in nature.

I'm also skeptical that there are so many "sleeper" Islamofascist agents waiting to be activated, but a bit less skeptical. The fact is, Islamofascism remains a highly divisive, diffuse, uncentralized, panoply of small groups and even independent individuals. Yes, there may be a couple thousand prospective "sleepers" in the U.S., but without any coherent central command and control what exactly are they going to do? If there is any nation or other entity with some promise it would seem to be Iran, and I agree that, it would seem to be a highly motivated goal for the current leadership in Iran to carry out such a strike on the U.S. If it was 2003 or even 2005 maybe I'd go along with the concerns you express with less skepticism, but it is nearly 7 years since the major hostilities of the Islamofascist War began and we've seen ZERO activity on the part of them in the U.S. It would be intersting to know how many attacks or planned attacks have been thwarted.

My analogy is important because it points out the basic truth: Iran can only argue for the legitimacy and expediency of a nuclear program by virtue of positioning itself as an adversary of the U.S. and her allies, unlike say Canada, Italy or Costa Rica, which are friendly and unconfrontational with the U.S. I'm sure many nations have a grievance or 30 with the U.S. and that they pursue resolution of those conflicts through a variety of public relations, legal, and diplomatic means, but they do not define themselves in terms of an inevitable clash of diametrically opposed moral opposites with the clear explication of inevitable violence.

In short, resistance is futile; building some piddly nuclear bombs to sit on top of some piddly short range missiles is both silly and futile. Iran does not have the wherewithal to win the long-term struggle against globalization. China and Russia have given up on playing the role of "anti-Capitalist" anti-American foil, what sense is there in some dinky, backwards nation like Iran or Venezuela or RNK acting as the bull dog for these lost causes? To be sure, the Russians and the Chinese are not major inner-circle allies, but they have also loosened up on their rivalrousness. They realized they couldn't beat "us" (or more precisely the consumer and civic forces of globalization) so they have by-degrees "joined us." Iran, Venezeula, RNK, and other hold-out numb-skull states need to wake up and smell the coffee: you cannot beat us, might as well 'join us,' like just about everyone else.

If we're gonna get on with establishing the United Federation of Planets, we need to start making progress toward that an Earth with ruled by one-common government and we only have a couple centuries to get on track with the Roddenberry schedule.

Moreover, the conceptual framework of the U.N. is oxymoronic. The putative primary purpose of the organization is to promote common welfare of humanity, but the secondary (and implicit?) structural basis for doing this is to entrench sovereign boundaries through the idea that "all nations have equal rights to sovereignty?" I'm not even aware of this being one of the explicit principles in the U.N. because if it were, it would promote secessions. Imagine Renaissance Italy . . . we do not need to be going back in that direction.

Moreover, even if such a concept of "every one who wants to have sovereignty deserves to have it" actually IS part of U.N. principle, it is contradicted by repeated actions of that organization. Imposing sanctions, embargoes, or simply writing letters to sovereign entities to "tell them how angry we are with them" is a fundamental contradiction of the idea that they can do whatever they want as a sovereign!

I find it hard to imagine the negative consequences if all aspects of the U.N. except the international dialogue, and the forum for integrating humanitarian efforts were scrapped.

ADDIT: on the point of "To say to another nation that you can't have peaceful nuclear reactors, but when oil prices impact us, we freely talk about building more of our own... that is also a double standard;' my understanding is that the specific type of nuclear facilities which the Iranians are striving to put into operation are NOT useful for nuclear power. They are ONLY useful for producing material for nuclear explosive devices. As such, any reference to such a double standard is a straw man comment/argument that can only serve to distract attention from the real issue: Iran is striving to build nuclear facilities that are ONLY suited to produce nuclear weapons. Many international, and national entities (the EU, NATO, the UN, and several nations including the U.S., but excepting China, Russia, RNK, Pakistan, and Venezeula to name a few rogues) have stated explicitly and repeatedly that the do not approve, and vigorously insist that Iran discontinue these efforts to create such nuclear facilities. In Civ4 terms: "the world" thinks the current Iranian leadership is a "villain."