This Guardian report says that North Korea would hit the United States with a "fire shower" of nukes if we attack first. But how far can NK deliver the 5-7 nukes that they're currently suspected of having?
According to Wikipedia, it depends. If they're using the short to medium range missiles, it's probably going to be the Musudan, which has an operational range of 2500-4000 kilometers. The darker red circle shows the maximum range of this. Nowhere close to US territories, but China, Russia and Japan should probably keep an eye open.
However, if they're going to use their Taepodong-2 long range three-stage missiles, there's a MAXIMUM possibility of 10,000 kilometers (the larger circle). However, a more likely scenario is somewhere around 4500 kilometers, which still isn't quite far enough to hit Alaska or Hawaii.
In the case that somehow it can reach the maximum 10,000 kilometers, it's still not quite there to the west coast of the US, so I can take my diapers off and stop worrying. [Guardian]
Update: Ah crap. There's a reason why I don't do this for a living. The Earth is round. Here's a better map. We're all screwed. Thanks commenters. Forget everything I wrote.