My biggest fear is that nuclear material sufficient to build a nuclear weapon will fall into the hands of a stateless terror group. IMHO, this is the scenario the Bush Administration should be working hardest to prevent. It's one thing for nuclear proliferation to continue among governments of established states. This is scary enough, especially in the cases of North Korea and Iran. At the same time, a state's acquisition of nuclear technology doesn't equate to the end of the world as we know it. To an extent, nuclear proliferation among established states can be handled by continued enforcement of nuclear non-proliferation agreements, negotiations and international pressure via inspections and other means.
But if a stateless terror group gets its hands on a nuclear weapon, we're all in a shitload of trouble. It's conceivable that something like the Pakistani/Indian conflict over Kashmir could one day produce a nuclear disaster, but negotiations and political pressure can help to keep this from ever happening. On the other hand, if nuclear material ends up in the hands of Al Qaeda or some other terrorist group, you can bank on it being used. And the city where I spend the vast majority of my time would be among the top targets for a nuclear attack by a rogue terrorist group.
My friends and I joke about "when it all goes down" pretty frequently, but as much as I'd like to tell myself that it's just a joke, there's legitimate fear there. I don't want to sound like the next militant crazy guy who arms himself to the teeth and builds a bomb shelter in his back yard, but I believe that a nuclear attack by terrorists is a very real possibility.
Consider that Pakistan's A.Q. Khan might never have been caught had U.S. intelligence failed to intercept shipments of material designed for making nuclear weapons as they were bound for Libya. Consider also that we may never have learned of the extent of the damage done by Khan had Khadafi not come to his senses and realized that giving up his nuclear weapons program was the right thing to do, given his situation. We can verify that Khan is probably the biggest figure in nuclear proliferation since the Cold War, having had dealings with North Korea, Iran and Libya (and these are just the dealings we know about and can verify).
Even if, as the Bush Admininstration has claimed, we have dismantled Khan's network, I think it's only the tip of the iceberg. Khan has shown that parties who really want nuclear weapons can piece together what they need via the international black market and that there is no shortage of willing providers of missile technology, centrifuges, designs, raw materials and everything else that is needed to make a nuclear weapon. Even if Khan's network has been dismantled, how easy would it be for the black market to compensate? I can't swallow any suggestion that the black markets have been shut down with respect to trading in materials that enable the construction of nuclear weapons.
Libya was caught with component materials, including plans for building a half-ton weapon that could fit into the back of a truck. North Korea probably already has a sizeable nuclear arsenal, as does Iran. What makes us think that all this quiet proliferation is limited only to established states and not to terrorists? All that one needs is money, the right contacts and a way to traffic the components undetected.
How far away do you really think we are from seeing the day when a stateless terror group builds a nuclear bomb?