The US House of Representatives Committee on the Judiciary went to court to compel Harriet Miers to comply with their subpoena. Harriett Miers claimed that she’s totally immune from Congressional Process, because she was working for President Bush.
The United States District Court for the District of Columbia just sided with the Judiciary Committee and struck down the Absolute Privilege defense to the Committee’s subpoena.
The District Court’s Memorandum Opinion is well worth your reading. While you’re at it, read the cases cited in the opinion: Marbury vs Madison, from 1803, US vs Nixon, from 1974, and Boumediene vs Bush, from 2008.