Federal courts apply federal privilege law in federal question cases, and state privilege law in diversity cases. Patent cases present a more complicated choice of law issue, because the Federal Circuit applies (1) its own law to patent issues and (2) regional circuit law to non-patent procedural issues.
In MPT, Inc. v. Marathon Labels, Inc., No. 1:04 CV 2357, 2006 U.S. Dist. LEXIS 4998, at *8 (N.D. Ohio Feb. 9, 2006), the court applied this basic principle — concluding that “the existence of the privilege will be determined by Federal Circuit law while waiver and the community of interest doctrine [usually called the “common interest” or “joint defense” doctrine] will be decided by Sixth Circuit law.”
Lawyers litigating patent cases should be prepared to deal with these and other subtleties.