True, but as CalMeacham said above, if you were trying to smuggle a nuke in you’d surely try to blend it in with normal sea traffic. Normal boat, normal shipping lanes, expected cargo (on paper, at least). A few miles out from the coast a speedboat dashes out to collect the crate and the ship continues into port.
Delivery is probably even more simple (simpler? sounds wrong for some reason) than most people here are assuming. There are plenty of import/export companies around the world, yes? There should be at least a few that are lax about internal security and self-regulation. One would merely have to get the bomb shipped via a third party and picked up here by the local chapter of your respective terrorist organization, all in a seemingly legitimate transaction (so long as no one opens the box). Hell, you might even be able to FedEx it if it isn’t too heavy. No need to sail up in a ship marked “Big Benign North Korean Freighter Full of Donuts – Move Along, Nothing to See Here.”
As SPOOFE points out, the really tough part is acquiring the bomb without anyone (us, basically) finding out. If we did find out about it, getting it here – one way or another – would be much more difficult (though by no means impossible).
Another concern is an anti-missile system’s destabilizing effect throughout greater Asia. China would obviously and easily counter the U.S. system by quintupling their nuclear arsenal, forcing India and Pakistan to do similarly. Japan’s conservatives would pressure the government to go nuclear (despite the constitutional prohibition), and Korea would suddenly find itself feeling especially vulnerable and needing to prove that it is a regional player. Russia, meanwhile, would suddenly feel ill at ease with these new threats, etc. etc. (China would also steal the technology and exploit its vulnerabilities.) Suddenly, the U.S. has squandered tens of billions of dollars on a useless, destabilizing system and does not have enough money to strengthen our conventional forces.
I think most posters agree that such a system would be great if it really worked, but it would also make the world a far more dangerous place–although such a prospect itself may be unavoidable. Any defensive system as fragile and primitive as the one envisioned can be easily overwhelmed, rendering it worse than an expensive boondoggle, as it actually increases the danger to the U.S.
I fear the 21st century will be far more dangerous than the 20th.