Courts disagree about who has the burden to establish that the attorney-client privilege has not been waived. Most courts hold that the party asserting the privilege protection must prove non-waiver if called upon to do so—by establishing that a privileged document has not been shared beyond the attorney-client relationship, etc.
One court taking this position recently contrasted it with the work-product protection. The Court in Western Resources, Inc. v. Union Pacific Railroad Co., No. 00-2043-CM, 2002 U.S. Dist. LEXIS 1911 (D. Kan. 2002), held that a different rule applied in the case of work-product protection, requiring that the party challenging the protection must prove waiver.
Such subtle distinctions between the attorney-client privilege and the work-product doctrine highlight the importance of lawyers becoming familiar with the nuances of both protections recognized by the pertinent courts.