The American Historical Association (AHA) is encouraging graduate programs “adopt a policy that allows the embargoing of completed PhD dissertations” – for as many as six years!
Here’s a link to the AHA’s statement explaining its rationale: http://blog.historians.org/2013/07/american-historical-association-statement-on-policies-regarding-the-embargoing-of-completed-history-phd-dissertations/
A rebuttal from Rebecca Rosen in The Atlantic: http://www.theatlantic.com/technology/archive/2013/07/american-historical-association-universities-ought-to-embargo-dissertations-from-the-internet-for-6-years/278024/