The United States Court of Appeals for the District of Columbia heard arguments on whether Office of Legal Counsel opinions on the scope of the FBI's surveillance authority. The Washington Post has more here.
Basically the question in this case is whether OLC advice is pre-decisional recommendations or is it a final legal opinion relied on by another agency (here the FBI). If it is pre-decisional, the recommendations may be protected by the deliberative process privilege (and possibly attorney work product and attorney-client privileges). However, if it is a final legal opinion it may be construed as working law and if so, case law would say it can't be withheld pursuant to a privilege. In other words, the government can't rely on secret law to justify its operations.
The Court should have an opinion in a couple of weeks.