According to the Washington Post (nod to Volokh):
An internal watchdog office at the Justice Department is investigating whether Bush administration lawyers violated professional standards by issuing legal opinions that authorized the CIA to use waterboarding and other harsh interrogation techniques, officials confirmed yesterday.
Most of the process won't be transparent to the public, so the only source of information about the precise grounds for the investigation may be leaks. The article says that a public summary may be realeased at the conclusion.
We've had discussions here and have cited to David Luban, Kathleen Clark, Stephen Gillers, and others, about the possible bases for ethics charges against OLC lawyers (independence and candor of advice; competence, etc.). I imagine that the recent Goldsmith book provided some grist for the investigations.