Another blog had a great rundown on the history of the sorts of attacks North Korea has done in the past:
The investigation into the sinking of the Cheonan is increasing pointing to a North Korean attack as the most likely cause of the tragaty. If so, it will not be the first or worst North Korean provocation.
The remarkable thing is how often these incidents, each of which would constitute an act of war* in other parts of the world, did not produce retaliation from Seoul or Washington. Here is a partial list:
1967: The warship Dangpo is sunk by North Korean artillery near the Northern Limit Line (NLL), which marks the de facto sea border between the Koreas on the West (Yello) Sea. 79 sailors die.
1968: North Korean Commandos infiltrate South Korea to assassinate president Park Chung-hee. They make it within a few hundred yards of the presidential Blue House before they are stopped. South Korea trained a group of convicts to launch a retaliatory attack but were never sent (as depicted in the movie Silmido).
Those incidents took place within the 1966-1972 DMZ war, a low-intensity conflict North Korean leader Kim Il-sung started to take advantage of America being engaged in the Vietnam War. President Park pushed for retaliation but was restrained by a Washington that wanted to avoid a larger conflict. The incidents continued after the "Second Korean War" petered out:
1974: A North Korean agent attempts to assassinate Park. He fails but kills Park's wife.
1983: North Korean agents attempt to assassinate South Korean president Chun Doo-hwan by bombing a stage in Rangoon, Burma (now Myanmar). Chun is not harmed but several South Korean ministers and about a dozen others die.
1987: Two North Korean agents destroy KAL flight 858 by bomb, killing all 115 people on board.
1999: North Korean boats repeatedly violate the NLL, causing exchanges of fire.
2002: Four South Korean sailors and an estimated 30 North Koreans die in firefights south of the NLL
2009: Yet another exchange of fire along the NLL
There have been few consequences for Pyongyang for engaging in those actions (although they have generally gotten the worse of the naval engagements along the NLL) and we can expect them to continue such provocations until there are. President Lee has to decide is now the time to try to break North Korea of the habit.
*Yes, the Koreas are still technically at war so North Korea's provocations could be considered ceasefire violations rather than acts of war.