Since the possibility of this has now significantly increased, we should start looking at the ramifications.
Nukemap is a very simple, useful tool that allows you to do some basic modeling of damages and casualties that would result from the detonation of a nuclear weapon. Now that the Obama/Clinton/Kerry brain trust has virtually assured that nothing will prevent Iran from developing nuclear weapons, it is time to look at what we might face.
I assumed a 20 kiloton weapon brought into a US port aboard a container ship. The yield is just a SWAG; halfway between the largest device detonated by North Korea and the 45KT Pakistani weapons. As a frame of reference, 20kt is roughly the yield of the Fat Man weapon used on Nagasaki. The actual effects would depend on a number of variables. The detonation would be at or near the surface, so the blast damage would not be as widespread, but the fallout would be greater. You can get more detailed information on the blast, radiation and thermal effects on the Nukemap page. The casualty estimates are based on daily averages of population within the range of the weapon. Actual number of deaths and injuries might vary significantly with conditions, just as happened in Japan. The Hiroshima bomb had a lower yield, yet caused more damage and casualties due to target geography and weather. Without knowing much about the technical capabilities of the Iranian nuke program it is impossible to make anything other than a SWAG about the form factor/size of the weapon. Nuclear weapons can be made very small and easily transportable, but that takes a degree of skill the Iranians may not have. The Hiroshima bomb, Little Boy, was very large and crude, but it was still able to achieve a 15kt yield. Still, it's size would make it extremely hard to transport into the US undetected. That would probably rule out aircraft or land transport. A 20 or 40 ft. shipping container could hold a weapon like that. It would also allow for some shielding to attempt to defeat detection equipment at ports of entry. I don't know enough about the radiation levels produced by a weapon or the sensitivity of radiacs used by boarding teams to know if that would be effective. My gut feeling is that you probably cannot shield a nuke in a shipping container enough to make it undetectable. So I guess getting it into port would depend on whether the USCG board and/or scan every vessel.
Models for other ports
Could the Iranians get a nuke into a US port? That's the big question. I'm sure the government has looked long and hard at this question and has a number of countermeasures in place. I can think of a few unconventional methods that might be used to get a weapon close enough to do damage. The Iranians probably have as well. Could the Iranians succeed? That depends on how effective our government is. I'm sure they have top men on this. Top men! Probably the same guys who got the tip from the flight schools about the 9/11 hijackers. Or it could be the ones who invited Anwar al-Awlaki into the Pentagon for Muslim sensitivity training. Maybe it was they guys who dismissed Maj. Nidal Hassan's behavior and contact with al-Awlaki. Or the ones contacted by the Russians about the Tsarnaev brothers. Oh well, whoever they get on it will undoubtedly be top men.
It's not like the stakes are that high; right?
