I'm usually the first to reject the idea that nuclear terrorism is a likely threat. However, while extremely low probability it is high enough impact that thinking a little about worst case scenarios isn't time wasted. The Nuclear Security Summit makes these issues especially current. Leaping off of this NY Times article discussing how Pakistan fits into the dialogue about nuclear security. Pakistan, of course, is rather less stable than most of us would wish a nuclear power to be. However, they have a sufficiently professional army that I'm not that worried about any of the actual weapons falling into the hands of terrorists. Since it didn't happen during Russia's problems I think the likelihood of losing a weapon if things ever get bad in Pakistan, such as state collapse which I think is highly unlikely but ultimately the worst-case scenario, probably isn't really a threat either. Though it is of course important that discussions happen so if there is an unforeseen disaster some sort of mechanism is in place to secure these facilities.
What does concern me however is that nuclear proliferation seems to be an issue mostly in the world's not so stable countries (also, if anyone remembers Abdul Qadeer Khan there's other reasons to not be pleased about these countries going nuclear). If Iran gets weapons I don't think there is any real risk of using them, but that is one more country where the state isn't at its most stable. I'm also, perhaps irrationally, unsettled by the idea that several neighboring countries, none of them exactly best friends, will form a strip from Iran to China where each country is nuclear armed. In addition, two of these are not so stable and all of them border unstable areas such as Afghanistan. Definitely an issue we have to give some thought to in case any of these areas become more unstable, or if the instability from Afghanistan, or elsewhere, shows signs of spreading.
All in all, if anything nuclear proliferation so far seems to be associated with less violence, which is cheering. However, it doesn't seem to be well associated with stability so that the countries most prone to proliferation seem to be also some of the less perfectly stable nations is a cause for concern. Not for great concern, though I do wish it was Switzerland that had a burning passion for becoming nuclear armed and not Iran.