It appears that the South Korean government is throwing this out there as a trial balloon to judge public reaction:

Despite North Korea’s denial of its involvement in the recent sinking of a South Korean Navy warship, Seoul’s foreign minister said yesterday that bringing the matter to the United Nations Security Council is an option if Pyongyang was found responsible.

It is the first time a senior Lee Myung-bak administration official commented on possible retaliation against the North since the March 26 sinking of the corvette Cheonan in the waters near the inter-Korean border.

An initial examination of the wreckage last week showed that an external explosion sunk the ship. Defense Minister Kim Tae-young’s characterization of the incident as a “grave situation of national security” has fueled speculation about a North Korean involvement.

“Although I am speaking about a hypothetical situation, the first thing that we can think about is taking the matter to the United Nations Security Council, if the North was proven to be behind the incident,” Foreign Minister Yu Myung-hwan said on the KBS’s “Inside the Issues,” yesterday morning.

Noting that the UN Security Council has authority to review matters related to a war, Yu said a discussion at the powerful UN organ in charge of the international security and peace is possible. The two Koreas are technically still at war because the Korean War ended with an armistice in 1953.

The Blue House said yesterday President Lee will issue a special message to the nation today regarding the Cheonan’s sinking.  [Joong Ang Ilbo]

It appears that the Lee Myung-bak government is continuing to make a slow and deliberate case that the sinking of the Cheonan was caused by the North Koreans, which will then justify whatever response the Lee government responds with.  I would be surprised if anything militarily is done when there are still very good non-military options available for the South Koreans to respond with.