In the case of In Re William Lyon Homes Shareholder Litigation, 2008 WL 3522437 (Del. Ch., Aug. 8, 2008), the Chancery Court ruled that the "at issue" exception to the attorney/client privilege did not apply to the facts of the case, and thus the privilege was not waived, because the defendants specifically adopted a tactic and strategy to use objective, non-privileged data to address the causation issues in the case–precisely so that they would not be subject to the "at issue" exception.
The court emphasized that a corollary to that tactic is that they will not be permitted to use that privileged communication or any other privileged communications with their attorneys to establish their causation arguments in the case.
The court acknowleged that privileges can serve as a barrier to the "search for the truth" but that is not a justification to disregard the protections afforded by the privilege.
See prior related decision in this case by the Delaware Supreme Court under the case name: Alaska Electrical Pension Fund v. Brown, summarized here, for additional background details, and for the court’s reasoning in which it addresses the right to attorneys’ fees in the settlement of a Delaware class action by non-Delaware lawyers who prosecuted related claims in California but were not involved directly in the Delaware class action (at least initially).