Can an attorney refuse to disclose the identity of a client?

The principle of Baird was not that the privilege applied because the identity of the client was incriminating, but because in the circumstances of the case disclosure of the identity of the client was in substance a disclosure of the confidential communication in the professional relationship between the client and the attorney.... "[T]he authorities are clear that the privilege extends essentially only to the substance of matters communicated to an attorney in professional confidence. Thus the identity of a client, or the fact that a given individual has become a client are matters which an attorney normally may not refuse to disclose, even though the fact of having retained counsel may be used as evidence against the client.... To be sure, there may be circumstances under which the identification of a client may amount to prejudicial disclosure of a confidential communication, as where the substance of a disclosure has already been revealed but not its source."

Id. at 593-94, quoting from Colton v. United States, 306 F.2d 633, 637 (2d Cir.1962).