I've been reading a lot of criticism about various hold-ups in missile defense lately and about how Bush's withdrawal from the Anti-Ballistic Missile treaty was going to be the thing which would protect us from the North Korean threat. Not coincidentally, there's a fair amount about how Clinton supposedly dropped the ball and how we're in danger now because of Clinton.
I've always been a little surprised that anyone thought that missile defense in its present form was a reliably good idea since an episode on 60 Minutes a while ago discussing the actual effectiveness of the Patriot system in Desert Storm. On unsophisticated Scud missiles of the sort Hussein was throwing at us, they were fairly effective as a system. Only one missile got through and it was the result of a system clock which had been inaccurate.
What wasn't said during Desert Storm was that standard practice was to launch three or four missiles at any incoming missile. There's evidence to suggest that the Patriot interceptors were successful in destroying missiles, but not in destroying the warheads.
In the more recent Gulf War, they performed approximately as well though they were notably defeated by an anti-ship missile and were the subject of two instances of friendly fire.
The trouble is that multiple warhead missiles have real potential to confuse anti-missile systems. Consider the relatively trivial addition of decoys to that. It becomes a case where you have to assume that each warhead is a real missile without further intelligence. That information becomes harder to obtain in the absence of frequent missile tests as well.
So let's assume a relatively trivial launch of 10 3-warhead missiles. With the Patriot's standard operating procedure, you'll end up needing to fire four missles per warhead. So in order to handle 10 missiles, you end up needing to launch and coordinate 120 interceptors. Consider a larger launch with the addition of reasonably effective decoys and the numbers are wildly out of one's favor. Not to mention it's a whole lot cheaper to add a warhead (or a decoy) to a missile than it is to build the four you expect to need to kill it.
Beam weapons such as Boeing's airborne laser make it slightly more workable, though the technology is still a ways away from being practical.