North Korea is in a very good position to draw heat away from Iran. They have a lot of practice in doing so for the PRC for so many years.
It's hard to focus on the big sleeping dog when the smaller rabid one is trying to bite you.
Kim Jong Il knows very well that a nuke test can be set up in short order. Unlike a ballistic missile test, it's next to impossible to detect until it happens. He also knows that, despite how hard we've been trying to ignore him, the tension is already built up. It's just built up for Iran. He can steal that in a heartbeat, by testing a nuclear weapon.
That will not only change the nature of the debate drastically. It will also make his subsequent episodes of sabre rattling seem more ominous. When he launches his next missile over Japan, the pucker factor will be a lot higher.
So, I wouldn't expect Iran and the DPRK to do something simultaneously. I would expect the DPRK to show up spoiling the party, right when it looks like Iran is in it's hottest water with the international community. All Kim wants to do is maximize his exposure, and he'll do so by stealing from Iran. Which is nice, because Iran simply wants to stall for time anyway, and will be glad to let Kim steal all the heat he wants.
After the nuke test, when U.S. finally decides to close in on N. Korea, Iran could do something to draw U.S. focus away from N. Korea.