I commented on this before, in this thread or a similar one. Here are some points I think people are missing:
#1: While the US could crush North Korea like an eggshell, the problem is that they would take South Korea out at the same time. North Korea can't hit the US with nuclear missles reliably, but they can drop a nuke over the border with relative ease and it would be very difficult to stop. This is the major tactical concern, and one I think a lot of people overlook.
#2: People misunderstand what nukes are currently capable of, a lot of the projections on "fallout" and the like are largely based on projections that are decades old. There isn't even one kind of "nuclear weapon" anymore, they come in various sizes and are designed to do differant things. The US could put a couple of subs in the region and nuke North Korea out of existance, and probably wouldn't cause much in the way of damage to surrounding nations, but the odds of North Korea getting a shot off into South Korea is again a factor.
#3: Morality is an equasion, the US is one of the only nations that really tries to maintain a serious distinction between "Civilian" and "non-Civilian" targets. Crying about collateral damage is one of the biggest ways to hamper the US war machine and cause damage and strife in the US. If the US *DID* employ WMD on North Korea people would be screaming about the millions of civilians that died in the course of the act, and comparing Obama (or whomever is at the helm) to Hitler. I suspect this would include people in this thread who are saying the US should crush North Korea. This is stupid due to the problems North Korea and it's culture represent, but that doesn't make it any less true. The non-military backlash from killing them could in many ways be as destructive to the US as if we had actually been hit by WMD. With the nation polarized more or less 50-50 and scholors around the world freaking out because we haven't collapsed into a civil war, an event like this could very well be what starts one. It would be THAT contreversial a move.
#4: Despite the way it looks at times, North Korea is functionally a puppet/proxy of China when it comes to this. As a lot of analysts have pointed out Bill Clinton lost, or sold military tech to China, one specific item mentioned was missle guidance systems. One of the big concerns being what China would do with this tech. A few years later, lo and behold, North Korea is unveiling an unprecedented amount of range on their WMD even if it's not quite long enough to reach Hawaii and other parts of the US. Meaning that China is likely to have sold or given this technology to North Korea.
With North Korea's beligerance, and the way they hold South Korea hostage, this makes China more nessicary to the US as a mediator, as well as ultimatly lets them use North Korea as a sock puppet in the region, while everyone pays attention to the obvious crisis going on with the insane "God Emperor", less attention is paid to things like Chinese anti-satellite systems, a massive military build up, and other things. North Korea threatening to use nukes pretty much keeps attention and resources focused on them.
This point is very debatable, but this is very much how things look to me. While it does get covered, the US and it's allies are spending most of their time and resources dealing with the "Axis Of Evil" which are all third world countries, while totally overlooking the obvious, rising threat.
#5: When it comes to Pakistan and India I have mixed opinions about the whole thing. The UK warned us from the beginning that we should be careful about who we allied with in the region, and allying with Pakistan pretty much wound up biting us in the keister.
To be entirely honest while India does seem a bit more sane, I'm not exactly comfortable with either of those nations having access to the technology that they do. Whenever India seems to have it's act together I run into some article about "The Temple Of The Rat" or how there is a city where Monkeys are sacred and allowed to run free through the city despite massive disruption, or how there are hundreds of thousands or millions of people making pilgrimages to watch a statue apparently drink milk.
Right now India's mystical/spirtual fervor is not directed at the US in anything like a negative fashion, but it still exists. The potential being just as bad there as it is with The Muslims, perhaps even worse in some ways.
Given the option I'd want to pull the WMD teeth of both countries to be entirely honest. I don't mind faith, I'm a Christian myself (albiet not a deeply spiritual one), but when you don't have a clear seperation of church and state, and weapons of mass destruction I think there is an issue.
These are my thoughts on the subject. The big point is that while I'm a militant there are a lot of reasons why the US hasn't taken direct action. Truthfully I think we could be handling the issue a lot better, and I think with some effort we could do a lot to reign in our morality for when we have to make hard desicians like this, but as things stand now the bottom line is that North Korea is no direct threat to the US, but is a direct threat to South Korea.