Here is an interesting article on the federal law of privilege.
Privileged evidence is not discoverable. Rule 26(b)(1), Fed. R. Civ. P. (limiting discoverable evidence to what is "not privileged"). Evidentiary privileges apply "at all stages of all actions, cases, and proceedings." Fed. R. Evid. 1101(c). "The policy behind extending privilege law to all proceedings is that the values protected by privileges can be destroyed by permitting disclosure of privileged material in any judicial context." WRIGHT & MILLER, FEDERAL PRACTICE & PROCEDURE § 8076. "Neither a subpoena duces tecum, nor any other procedure in a civil action, may be used by a party to obtain privileged documents." Id. at § 2458.
The Federal Rules of Evidence determine what is “privileged” under Rule 26(b)(1). See, e.g., Matter of International Horizons, Inc., 689 F.2d 996, 1002 (11th Cir. 1982); Armour Intern. Co. v. Worldwide Cosmetics, Inc., 689 F.2d 134, 135 (7th Cir. 1982). Under Fed. R. Evid. 501, federal law apparently governs privilege except on state law claims.