Atlantic Council

Speculation about Kim Jong-il's health is one of a few worrying developments to come out of North Korea recently.  The Independent described the North's intentions to close its border with South Korea, its largest provider of aid:

The world's most heavily guarded frontier, the line that divides North and South Korea, is the focus of renewed tensions with confrontation looming on several fronts.  From 1 December, the secretive North Korean leadership will close the land border between the Koreas at the few places where there are still openings.

Whether Pyongyang stops accepting the aid, despite the closed border, is another story.  And the North's response to its removal from the U.S. state sponsors of terrorism list?: "Pyongyang is also stopping UN nuclear inspectors from taking soil and nuclear waste samples from its nuclear plant at Yongbyon."

Very comforting.  Even more worrying is the thought of what might happen if Kim's health truly is as dire as many now believe it to be.  Intelligence reports since September have suggested that Kim is, at the least, recovering from a stroke (or two).  At this point, the North's constant reassurances that Kim is healthy, based only on questionable photographs, is arguably doing the government more harm than good:

Its leadership, mysterious in its composition and strategy, has few cards in its hands.  International attention focuses on its nuclear weapons, primitive though they may be.  But without its nuclear program the North, impoverished and ruined, has no bargaining strength.  It may see the election of Barack Obama in the U.S. as presenting opportunities that are not there.  Its threats carry a certain weight because it is too weak to retreat from them.

The Globe and Mail paints an equally scary picture:

When he visited South Korea a few years ago, Jang Song-taek was sometimes seen in his pajamas, wandering the hallways of his hotel, always followed by an entourage of obsequious underlings.  When he overslept one morning, nobody in the North Korean delegation dared to knock on his door. Instead, they begged a South Korean to rouse him from his slumber.  The eccentric Mr. Jang, still a mystery to the West, may now be the most powerful man in the Hermit Kingdom. The 62-year-old brother-in-law of Kim Jong-il is said to be running the day-to-day affairs of North Korea after Mr. Kim reportedly suffered a serious illness and perhaps a series of debilitating strokes.

[...]

Mr. Kim has never designated an official successor.  His three sons are seen as too young or too error-prone.  ...  In the absence of any official heir, Mr. Jang is seen as the likeliest successor and possibly already the behind-the-scenes power.  One report said he has consolidated his power by gaining control of the North Korean domestic security agencies.

The article also addresses the border closing, calling it a sign of possible instability:

The fear of political turmoil is apparently the main reason for the recent closing of North Korea's border with China and the deployment of Chinese troops to the border zone. Pyongyang has also announced that it will close its borders with South Korea next month, leaving the country even more isolated.

See the bottom of the Independent article for a timeline of the reporting on Kim's health.