north korea is only using this as a "threat" to try and scare the south koreans into resuming the food supplies that they were sending into the north. the north koreans will not do anything, they know that they have no backing from china, russia, and obvisouly the united states.
China wont back them, because they know that in doing so would destroy the fragile trade relationship between themselves and the USA, russia wont back them becasue they no longer are a communist country, and are trying to join NATO, and nato wont let you join if you back a non-nato country/declare war on a NATO country.
So north korea are trying to use dumbed down coldwar tatics of fear into forcing some one to do something. thats why there hasnt been an invasion and only firin across the border.