Nothing is impossible about putting an existing warhead on a truck and driving it to a wharf for Al Qaeda to collect.
While its unlikely a terror state could be stable enough for PRODUCTION of nuclear weapons, the likelihood of a terrorist outfit being gifted the PRODUCT is quite high, as it is the one way enemies of the American regime can employ such weapons against the American target without any fear of retaliation. Especially our fleet in the Persian Gulf, which permits unknown vessels to get within machine gun range. The world knows--through American experimentation--what happens to even the largest warships a few miles from a submerged fission explosion.
There is no uncertainty about AQ attempting the use of an atomic bomb with skilled, trained professionals willing to die in the attempt. The only uncertainty is whether regimes possessing nukes hate us that much.
The core joke of Hitchhiker's Guide to the Galaxy is that of course no civilization would develop personal computers with instant remote database recovery, and then waste this technology to find good drinks.
Steve Jobs has ruined this joke.