Time has a 'exclusive' feature this week on Operation Historic Protector, an effort launched by NARA's Inspector General last month "to combat what many fear is a growing threat to the federal government's historical repository, as well as to state archives and university libraries: the pilfering of old letters, documents, maps, photographs, books and other historical artifacts."
Right now the project is pretty low-level, with two staffers from the IG's office assigned to monitor the manuscript trade in search of suspicious items. Paul Brachfeld, the inspector general, told the magazine that OMB has so far refused his request to fund the operation, "but he says he hopes to build up his force along with a network of outside artifacts experts around the country who will tip off his agents 'every time they find something suspicious. And we swoop down.'"
Brachfeld told Time that his office is investigating a "major case" of document theft "in which 'almost a hundred documents' are believed to have been stolen by a National Archives employee. Brachfeld would not discuss details of that case because 'it is awaiting prosecution.'" This one has not hit the news so far as I know.
In the wake of the Smiley/Berger/Harner/&c. cases in recent years, this doesn't seem like a bad idea. When I was at Union we recovered multiple items stolen from the library by monitoring eBay, so it certainly can be done. Clearly we can't just sit back and ignore the problem.