Atrios wonders what would be the correct response to nuclear terrorism against an American city. I think the answer is self-evident: we have a nuclear deterrent for a reason. If we are unwilling to use that force to respond to an attack on one of our cities, what good is it? Would our silo's silence not destroy any deterrent value our ICBM fleet carries? Our response should be to quickly determine the point of manufacture and launch massive retaliation.
Of course, nothing involving the killing of millions is ever that simple. The science behind this is not nearly as easy as the "nuclear fingerprint" nomenclature implies. Attribution Technology, it is called, and it is still an imperfect science. There are certain sources that have particularly distinctive characteristics, but an attack might well remain anonymous if we are doubly unlucky.
To engage in some cynicism: What are the odds that because of political pressure we would declare a source when the science wasn't certain? I hope we would show the moral restraint, but we have a government of men, and I fear their weakness before the rage of the American people.
In reality our nuclear deterrent has always been a psychological one, since the use of even a fraction of the world-wide arsenal would bear enormous implications for the planet's habitability. So, because terrorists would deliver the bomb via a non-ballistic method (i.e. no easy-to-read return address), our deterrent needs to change shape. Perhaps a world-wide public information campaign along the lines of "We have the technology to tell for sure where a bomb was made, and that country will be committing national suicide if one of its bombs is used in an attack." Even for a group like Al-Qaeda to whom friendly civilian deaths are a strategic victory, the potential genocide of their people must still be a deterrent... right?