North Korea is a threat to the United States more indirectly than directly in my opinion. South Korea is in the greatest danger from the north right now due to Kim Jong Il no longer recognizing the armistice that was established in 1953.
As South Korea is a strong ally of the United States and 32,000 of our troops are stationed there, any attack on the south is a threat to that country, interests we might have there, and to the thousands of American servicemen stationed there.
North Korea has missles capable of reaching and purportedly being able to destroy United States territories in the Pacific Ocean along with Hawaii. These are certainly significant threats to the United States but I don’t think North Korea has the fortitude to launch against us because they know that our retaliation can and probably would be very much worse right back at them. Additionally, North Korea isn’t going to get any real support from China and Russia like they did in days gone by and I think they know this too.