A good-sized fishing boat would work well.
Agreed. Just stick a Scud on a fishing boat or garbage scow owned by some Liberian or Panamanian front company, and you pass under the radar (literally and figuratively).
I am, however, far more worried about the Ragheads exploding such nukes 150-250 miles up and destroying our electrical and digital infrastructure via EMP. That'd kill far, far more people (up to 90% of the population within a year due to starvation, disease and fighting over food, etc.) than directly nuking a city, and would have the added advantage of destroying us as a power (let alone a superpower) forever. EMP is the ultimate in assymetric warfare.
No one knows how an EMP explosion would work—would it really take out a whole section of the nation or would it be spotty—some with power, some without. Would it be a return to the 19th Century? Also, what about our ships at sea and subs? Would they launch at any attacker—we would know from where it came and blast with H-Bombs any nation that attacked us—maybe take out others as well. We have B-52s, B-1s and all sorts of rockets. I would doubt if any nation would risk the fall out of such an attack. Iran would simply ceast to exist—maybe take out Pakistan and North Korea to boot.
The nukes the Iranians are capable of building in the short term won't do the trick for widespread EMP. To really nail large chunks of the US you need multi-hundred KT warheads. All this is not to say we shouldn't worry longer term!
Here's a good link regarding Soviet EMP testing over the mainland USSR(!).