As for actual evidence? It is pretty thin. As I noted at the time Iran and North Korea signed the MOU, North Korea's state-run media outlet, KCNA, released the list of attendees. The fact that the heads of the Defense Ministry and the Atomic Energy Organization of Iran attended caught my attention -- but it is hard to say more than that. One Iranian defector -- and we ought to treat such sources with considerable skepticism -- has asserted that North Korean specialists visited Iran to assist in the "nuclear program," although he did not make clear in what capacity or how he even knew their function.
Perhaps the most reliable report comes from Paul-Anton Krüger in the Süddeutsche Zeitung. "Western intelligence sources" told Krüger that North Korea transferred a U.S. computer code that models radiation transport, MCNPX, to Iran.
And that's about it. Is there cooperation within the centrifuge programs? Might it extend to warheads? Maybe. Hard evidence is in short supply. Japanese and South Korean officials have repeatedly asked their U.S. counterparts whether nuclear cooperation exists between North Korea and Iran. You can read the private remarks behind closed doors thanks to Wikileaks: In December 2007, Japanese Assistant Chief Cabinet Secretary Hiroyasu Ando asked acting-Undersecretary of State for Arms Control and International Security John Rood about cooperation between North Korea and Iran on nuclear weapons: "Acting U/S Rood indicated no evidence had been found of exchanges on nuclear or associated technology between the two countries despite longstanding and ongoing cooperation on missile development."
The MCNPX episode might qualify that statement a bit, but we're still just guessing.
It is actually kind of interesting to imagine what might limit cooperation between Iran and North Korea. One factor may simply be North Korean enthusiasm for secrecy. Leslie Groves has nothing on the North Koreans when it comes to keeping tabs on nuclear scientists. When Burmese dissidents obtained a report detailing a trip by military officials to North Korea, Pyongyang reportedly leaned on the Burmese junta to execute the leakers. And A.Q. Khan -- who can be counted on to save his own skin -- seems to have gone to extraordinary lengths to protect his North Korean contacts, even while ratting out the Libyans and Iranians. That ought to tell you something. A cursory glance at the IAEA annex on "possible military dimensions" of Iran's nuclear program, on the other hand, suggests the Iranians aren't much on security. I imagine the North Koreans are not terribly eager to stick any Iranian thumbdrives in their computer systems. And if North Korea did transfer the MCNPX code to Iran, I am certain that the cadres in Pyongyang did not enjoy reading about it in the morning paper. The Iranians may share "the common idea of anti-imperialism and anti-U.S. struggle," but they are still foreigners.
Then there is the question of whether cooperation would, in the grand scheme of things, make much of a difference. Indeed, if we think about Iran's nuclear program and the role that nuclear testing might play, showing up at a North Korean nuclear test is probably more about sticking it to the man than anything else. At the end of the day, a North Korean test is simply not the same thing as an Iranian test.
Let's take another look at the Iranian nuclear program. The view of the U.S. intelligence community, which I continue to think is the most reasonable view, is that Iran pursued a covert weapons program before pausing it under international pressure in 2003. Iran could restart the program at any time, which creates an interesting policy problem: An attack on Iran might destroy some capabilities and kill some scientists, but it would result in Iran pushing "play" on a bomb program. That's not a sensible thing to do as long as we have confidence the program remains paused.
Some people are claiming that a North Korean test is also an Iranian test for the same reason they claimed Iran had reorganized its Physics Research Center -- to assert that Iran has restarted its weapons program and that we must now move toward military action. In both cases, however, Iran is simply maintaining its nuclear weapon option. That's not a very happy thought, but it would be Iran's exercise of the nuclear option that sends us to the mattresses.