You're probably right there. I think the terrorist states of the world missed their window of opportunity. The US missile defense system, imperfect as it is, still under development, etc. is still improving in capability faster than the North Koreas and Irans of the world are developing missile and warhead technology. I think that's a race they've lost.
Now, smuggling a warhead or six into the US? I think that, while difficult, is probably far more feasible.