A couple of years ago, Stephen Gillers asked whether the duty of confidentiality prohibits a lawyer from responding to a former client's public criticisms. As Stephen's post suggested, the Model Rules do not clearly resolve the issue.
Now comes the Los Angeles County Bar Association's Professional Responsibility and Ethics Committee with a useful opinion on the subject (here). The opinion addresses the issue under the California Rules of Professional Conduct, which (at least for now) are quite a bit different from the Model Rules, but the analysis is informative. In a nutshell, the opinion says that a lawyer "may publicly respond to such comments as long as the rebuttal: (1) does not disclose any confidential information; (2) does not injure the former client in any matter involving the prior representation; and (3) is proportionate and restrained."
For the reasons referenced in the comments to Stephen's original post, I think the answer is (and should be) the same under the Model Rules.
(H/T to the always-useful ABA/BNA Lawyers' Manual on Professional Conduct. I think I've put in a plug for the Lawyer's Manual before, but it deserves another one. The publication offers the most effective and comprehensive regular coverage of professional responsibility issues I've seen. If you specialize in professional responsibility and don't subscribe, make it a holiday gift to yourself!)