The answer is probably no, despite Chinese microblog Weibo being lit up by the rumor that Kim has been assassinated.
The most convincing rumors are, of course, the ones that play on genuine concerns, and it’s uncertain how successful Kim Jong-il’s son will be in consolidating power at such a tender age and with no real military experience. So an assassination isn’t completely beyond the realms of possibility.
Interestingly, last month the Korea Herald noted that there “have been reports that Kim Jong-un had planned to assassinate his eldest brother (Kim Jong-nam) and that such plans were thwarted by China’s intervention.”
This kind of tension certainly chimes with the mood described by Scott Snyder, a Korea specialist with the Council on Foreign Relations.
He wrote last month: “Kim Jong-nam, aged forty, is Kim Jong-il’s child with his first wife, Sung Hae-rim. As Kim Jong-il’s eldest son, Kim Jong-nam is reported to have been groomed for succession until he fell out of favor in 2001, after being detained at Narita Airport in Japan with a fake passport. Since that time, he has lived in apparent exile in Macao and Beijing. Kim Jong-nam has emerged as a surprisingly voluble critic of North Korea’s leadership succession, directly challenging the legitimacy and capability of Kim Jong-un as a leader.”
I also asked Bryan Kay, The Diplomat's Seoul correspondent, for his take. “It would seem since his older brother spoke with Japanese media calling into question his credentials and staying power that any perceived chink in the North Korean leadership facade is prone to such rumors, true or not.”
So anything is possible, but we’ll need more than the notoriously unreliable Chinese “twitterverse” to go on.