The American Bar Association Standing Committee on Ethics and Professional Responsibility has issued a new opinion on the obligations of lawyers to return file/documents to former clients. Formal Ethics Opinion 471 is available here. (Get it now, because the ABA archives opinions after some time, after which they are only available to members.) For more on the opinion go to Professional Liability Matters.
UPDATE 8/16/16: Lawyers Ethics Alert Blog has a comment on the Opinion here.