If you were smart, and wanted to cause serious damage or casualties, nukes would be one of the worst ideas. And seeing as you need to be fairly smart to begin with to construct or operate a nuke, that probably means there won't be any terrorists nuking anything. They'd be more likely to poison the water supply, blow up a chlorine plant, or kill some people in the presence of the media (the effects of terrorism are much more damaging than the acts which caused terror). Simple things. Neither expensive nor difficult and only minimally risky. A nuke either must be stolen (risky and difficult) bought (expensive) or built (very difficult). It must be transported to where it will be detonated which is both risky and difficult, as the US government has placed geiger counters on many freeways, so transport could only be done on less common roads, and even then there is no guarantee the vehicle won't be detected. Going across states would also be rather difficult, but I suppose not impossible. Or, with almost no effort, they could plant a truck bomb at a chlorine gas plant, and the resulting gas could kill an entire city if the wind is blowing that way. Same damage, but with way less effort, risk, and financing. Incidently, that's why I think we should have greater security measures around potentially dangerous facilities.
In any case, it's highly unlikely that terrorists, or nutcases, or hate groups (?) would nuke us. But what about Russia, Iran, China, or North Korea? Well, that's where MAD comes in. No foreign ruler would send a nuke our way unless he wanted to die along with his entire family, all his friends, and pretty much everyone he's ever known when we retalliate. Again, it's not very likely. Indeed, a foriegn ruler could probably cause more harm more safely by just supporting groups that are already fighting us, such as Al Qaeda. They wouldn't get nuked, but they could still be indirectly killing us and blaming it on them.