Check out this awesome article in the Bulletin of the Atomic Scientists:
Nuclear Terrorism's Fatal Assumptions
This is the best piece I have ever seen on the subject. The problem of nuclear terrorism has been completely overblown, and most of the things "everyone knows" about the subject are simply wrong. Popular misconceptions about the potential misuse of ordinary civilian nuclear technology by terrorists, or the supposed ease of acquiring nuclear weapons, materials, and expertise in the former USSR, abound.
I'm planning to write a post about the usability of reactor-grade plutonium in nuclear weapons, and the reasons that I believe that the proliferation risks of this class of material have been grossly overestimated. The incredible thing is that a critical part of the debate hinges on an appeal to the authority of a single document produced by Sandia National Lab in the 1970s- a document that is probably being misinterpreted, at the very least. Besides this, there are excellent historical reasons for believing that this material is extremely difficult to use in nuclear explosives. I'll explain this in detail when I have time.