At the time of Kim Jong Il's apparent stroke in August 2008, his brother-in-law, Jang Song-taek (who is married to Kim's sister, Kim Kyong-hui), was rumored to be the key backer of Kim's oldest son, Kim Jong-nam.
In February 2009, Yonhap reported that Jang had shifted his support to Kim Jong-un in light of Kim Jong Il's "special affection" for his third son and successor, and out of consideration for his own future political power. Worried about being purged, as he was in 2004 for becoming too powerful within the regime, Jang agreed to throw his support behind Kim Jong-un. According to senior North Korean defectors in South Korea, Jang reached a deal with Kim Jong Il.
In return, Kim Jong Il has allowed Jang to engineer the succession by placing his allies in key posts throughout the regime. The new chief of the general staff, Yi Yong-ho, is allegedly close to Jang, as is the new minister of the People's Armed Forces. The NDC is now populated with powerful military and security officials with ties to Jang, including vice chairmen Kim Yong-jun and O Kuk-yol and members Chu Sang-song (minister of public security) and U Tong-juk (deputy director of the State Security Department).
Many think this collective leadership, which probably extends beyond the NDC to the party as well, will provide the support network for the dynastic succession.
In this scenario, Kim Jong-un would be the public face of North Korea, while Jang leads behind the scenes.
[Thoughts from Ken E. Gause, senior analyst at CNA, who has studied the North Korean regime for two decades]