White House bars agencies from posting some statistics, by Aliya Sternstein, NextGov (01/27/2010).

According to this article, datasets posted to data.gov by the Nuclear Regulatory Commission, the Peace Corps, the Agriculture Department's Food Safety and Inspection Service, the Interior Department's Bureau of Reclamation, and the Social Security Administration have been removed by the Office of Management and Budget "because they raised privacy, security or other concerns."

The article is based on work done by OpenTheGovernment.org which is tracking agency participation with the Open Government Directive here.