A quick history lesson for those of you not familiar with that landmark decision:
United States v. Nixon, 418 U.S. 683 (1974), was a landmark United States Supreme Court decision which resulted in a unanimous 8–0 ruling against President Richard Nixon, ordering him to deliver presidential tape recordings and other subpoenaed materials to the District Court. Issued on July 24, 1974, the ruling was important to the late stages of the Watergate scandal, when there was an ongoing impeachment process against Richard Nixon. United States v. Nixon is considered a crucial precedent limiting the power of any U.S. president to claim executive privilege.
Here is the rest of the Wikipedia entry.