Reply #15: Here's more info on what I think is the precedent.

[robertpaulsen]

Opinion
Written by: Chief Justice Warren E. Burger
Joined by: Justices William O. Douglas, William J. Brennan, Potter Stewart, Byron White, Thurgood Marshall, Harry Blackmun and Lewis Franklin Powell, Jr.
Associate (later Chief) Justice William Rehnquist recused himself, and did not participate in the trial.


The case
Following a subpoena of the Watergate tapes by special prosecutor Leon Jaworski, Richard Nixon sought to have them quashed on the ground of executive privilege. The Court ruled 8-0 that the tapes should be released.

The Court determined:

that the courts have the final voice in determining constitutional questions and
that no person, not even the President of the United States, is completely above law.
Most importantly the Court determined that a president cannot use executive privilege as an excuse to withhold evidence that is 'demonstrably relevant in a criminal trial.'

more...

http://en.wikipedia.org/wiki/United_States_v._Nixon