It is quite obvious that as long as there are people in the world and as long as no method of destruction is superior to a thermonuclear bomb, nuclear proliferation is a battle that can not be won. It can only be kept at bay, or lost. As times come and go, more and more people and groups (such as nations and terrorist organizations) will be able to obtain nuclear weapons or the blueprints and resources necessary for their construction. No developing nation will remain satisfied with the delegation of thermonuclear weaponry to only a few nations out of the more than two hundred currently present; those "nuclear powers" that have nuclear weapons as of current will become increasingly pressured to share their technology with other nations, under the guise of nuclear power.
The consequences of a terrorist organization or state obtaining nuclear weaponry is frightening, but in practicality ineffectual. This is due to the MAD, or Mutually Assured Destruction, policy, which states that no side will use a nuclear weapon because both sides will be instantaneously annihilated. There are no risks of nuclear winter resulting from sharing nuclear weaponry with "rogue states" such as Iran, Iraq, Afghanistan, and North Korea. As for terrorist groups such as Al-Qaida, nuclear bombs will prove paltry for their aims, primarily because, lacking a great number of warheads (>100), all projectiles will be shot down by defensive ballistics such as the United states Patriot Rocket.
Given these arguments, and a society increasingly rational and literate that will agree with these arguments, and the democratization of some of these "rogue states", nuclear proliferation will occur en masse, with the politicians of the current nuclear powers giving way to their opponents' arguments.