The U.S. Embassy in Seoul sent this cable to the State Department on February 18, 2010. It summarizes what Assistant Secretary Kurt Campbell learned from meetings with South Korean leaders and experts about the possibilities of succession in North Korea.
Summary from document released by Wikileaks, posted on National Security Archive:
A group of five ROK opinion leaders and experts on North Korea issues told A/S Kurt Campbell on February 3 it was difficult to predict whether Kim Jong-il's youngest son Kim Jong-un would be able to succeed his father without sparking instability in the North. Of the five experts, one thought the younger Kim might succeed and one argued his lack of leadership experience made it unlikely he would win the support of the ruling elites. They agreed that Kim Jong-il's brother-in-law Jang Song-taek would prove a strong rival for the younger Kim and would probably be tempted to challenge him. Kim Jong-il had used draconian controls and international aid to discourage coups after having foiled three such attempts in the late 90s. China's strategic interests were fundamentally at odds with U.S.-ROK interests in North Korea.