[Alberta's Child]

That was one of the key points of contention in the Barry Bonds steroid investigation. California has a “shield law,” but it has no relevance in a Federal case.

[Lurking Libertarian]

California has a “shield law,” but it has no relevance in a Federal case.

Never in a federal criminal case, and only occasionally in a federal civil case (under Rule 501 of the Federal Rules of Evidence).