In 2006, the Department of Energy formally declassified the already widely publicized fact “That the People’s Republic of China obtained some Restricted Data information on the W88 [nuclear] warhead, and perhaps the complete W88 design.”
Then, in a remarkable display of bureaucratic acrobatics, DOE classified the memo that authorized the declassification of that information. The declassification memo was found to merit classification at the Secret/Restricted Data level.
Five years later, in 2011, the two-sentence memo was reviewed for declassification and DOE has now released it.
As often seems to be the case, declassification here lags behind disclosure rather than leading it. For a convenient summary of issues surrounding China and the W88, see China: Suspected Acquisition of U.S. Nuclear Weapon Secrets, Congressional Research Service, updated February 1, 2006.