Technology Review has a worrying piece (at least for US readers) about how the fear of terrorism has led to increasing restrictions on foreign students and limiting the access of both foreign and U.S. citizens to various materials and lines of research.
While the article focus is mainly bio, we do hear anecdotal evidence that this is also happening in the nanotech field.
In the days of Oppenheimer, perhaps you could get away with hiding certain areas of research, but in the days of email and online journal publishing, it is very easy for the genie to escape the bottle, and the recent proliferation of nuclear technology has shown the difficulty of slowing the spread of knowledge, and even sophisticated equipment.