PNAS recently declined to publish a paper which constructed a model of a bioterrorist attack. The US Department of Health and Human Services expressed concerns over the paper, which they argued would be an excellent source of information for someone who actually wanted to stage a real bioterrorist attack. The paper was subsequently withdrawn. This raises some interesting questions, which are explored in a Nature editorial in the latest issue.
The BMJ of the 18th June also has an editorial.
The paper has now been published - see this report from Nature. The paper is here.