I've been mediating on the judgements made by the Japanese Ministry of Defense report in the article I translated on Tuesday. I agree with their view and here is my logic:
North Korean missile designers are resourceful, but their current string of designs lack originality. They nailed down the R-11 'Scud' design in the 1980s and have been producing it domestically for ever since. The North Koreans have even made a modest income exporting Scuds that have been modified to extend their range. The medium-range No Dong is often credited as an indigenous North Korean design, but most accounts of No-Dong development either describe it as a scaled-up Scud or a modified version of the ancient R-13 'Sark' submarine-launched ballistic missile.
Either way, North Korean engineers can only squeeze so much more additional range out by lengthening the missile and shaving down the payload.
This brings us to the Taepodong-1, North Korea's first attempt at building a multi-stage missile. The Taepodong-1's design is familiar enough -- a modified Scud-B affixed to the top of a No Dong missile. The only new component is a small, solid-fueled third stage (maybe an KN-02?) situated on top of the Scud B. When the North Koreans attempted to launch the Taepodong-1 back in August 1998, the first and second stages appear to have worked, but third stage did not.
The Taepodong-2 also represents a relatively bold step away from the previous incrementalism of the North Korean missile program. Not only that, it appears to be very different from its similarly named cousin. The Taepodong-2 launch last July appeared to be a two-stage missile -- a No Dong missile fixed to the top of what may have been first stage of a DF-3A. Whatever the first stage was, it probably need to go back to the drawing board because it flamed out after only 40 seconds of burn time last year.
Based on this track record, I feel pretty sanguine about the prospect of vertical missile proliferation in North Korea. Pyongyang's best brains were only able to crack the Scud formula after decades of incremental reverse-engineering (and probably a fair amount of trial and error). Attempts to incorporate new technology appear to be consistently (forgive the pun) blowing up in their faces. This gives me a pretty strong impression that we should be far more worried about North Korea's nuclear scientists than their missile engineers.