A client, not the lawyer, is the "holder" of the attorney-client privilege. By holding the privilege, the client alone can decide whether and to what extent a lawyer may share confidential details of the lawyer's representation of the client. There are some rare exceptions - among them is self-defense.
When a client files a grievance against a lawyer, the lawyer may breach some aspects of the attorney-client privilege to defend herself. If a client says, "My lawyer never returned my e-mails," the lawyer may share e-mails with the State Bar to prove that he was responsive to client demands. That rule is fair, since a client shouldn't be able to deny his lawyer the right to defend herself.
Enter the Internet.
What if a former client writes: "My lawyer was terrible. He never returned my calls or e-mails. I had a million-dollar case, and she blew it!"
Some prospective clients might read that blog entry, and thus never call the lawyer. Current clients might get nervous. Other lawyers might decline to refer a case to the bad lawyer.
Under the current Rules of Professional Conduct, it would certainly be unethical for the lawyer to write: "John Smith called me 5 times each day. He asked the same questions over and over again. After evaluating his case through discovery, we realized his case was marginal. We told him to settle the case for $25,000 - nuisance value. He refused. The trial court dismissed the case on summary judgment. Now he's angry. By the way, you can read the case filings here, here, and here to decide for yourself whether we blew a huge case."
Does that Rule make sense? A lawyer can lose business. Online reputation matters - not for a lawyer's ego - but for his business. The law offers trademark protection. A brand matters. A lawyer is only as good as her name. Shouldn't a lawyer be able to breach some aspects of the attorney-client privilege in order to protect her name?
Now, trashing a client would be a bad idea. As a perspective client, I might think, "Damn. I don't want that guy representing me. He's just going to bash me to make himself look better." So I can think of many reasons why it would be bad business to breach privilege.
My question is narrower: Where a client trashes a lawyer online, should that lawyer be allowed to break the attorney-client privilege (on scope, but not details of representation) to defend her reputation?