In a recent article, South Korea claims that evidence has surfaced over the last two years of enormous piles of earth appearing at former underground nuclear testing sites in North Korea. They take this as a sign that a third underground nuclear test since 2000 may be in the works. The others were in 2006 and 2009. Normally this would be seen as quite alarming. However, there are some theories that this information was released by South Korean intelligence in order to bolster support for the conservative party in elections just three days away. The National Intelligence Service is denying any political motivations behind this, but the timing is interesting.
North Korea is already on the hot seat for planning a missile launch in direct violation of an agreement for food aid from the U.S. If this evidence of additional nuclear testing pans out, it will stretch even more thin the relationship N. Korea has with the international community. However, this is a familiar pattern of behavior from N. Korea. They build up their nuclear program or test missiles, and then agree to stop in exchange for much needed food aid, only to break their promise and repeat the cycle. The new leader in N. Korea, Kim Jong-Un, is likely doing this to make his mark early as a strong figure at the top of the garrison state that is N. Korea. A relatively strong military is all that N. Korea has to go with, and leadership needs to maintain the image as such. With continued support from China, it is unlikely that we will see a shift in N. Korea's behavior any time soon.
South Korea and Japan have both stated that if the missile or any fragments of the missile approach their territory, they will shoot them down. North Korea has responded officially by saying that this would be interpreted as an act of war. However, it seems unlikely that Pyongyang would be willing to go to war over a single shot down satellite missile, especially if it malfunctions and veers off course towards Japan or South Korea. This type of belligerent rhetoric is typical of the Koreas. China has stated its displeasure with the planned missile test but has not issued any further statements regarding actions to be taken if the test goes ahead. China is unlikely to take official steps to reprimand or sanction North Korea.
EDIT: Thursday 4/12. North Korea launched their missile on Thursday and it was a resounding failure. This is not the first time their missile testing has had such an outcome. The rocket broke up just 81 seconds into flight and landed in the ocean. It is an international embarrassment for Pyongyang and the DPRK leadership. The UN Security Council met to condemn the test on Friday.