The Florida Bar Association has come down holding that it is unethical to "mine" documents transmitted by opposing counsel. The Board has stated that it is examining whether to adopt a rule or bar opinion making its position clear.
This puts both Florida and New York as saying it is unethical to examine embedded data in documents (obviously, we're not talking here about document production, but rather the e-mailing of documents such as drafts of contracts, etc., during negotiations. Lawyers probably have a duty to examine e-files for embedded data during production in some cases).
I believe they're right. I've spoken about metadata around the country, and apart from a few lawyers, many had no idea that Microsoft Word documents, Word Perfect documents, and other files contain hidden text, in some cases including past revisions to the document. But, 2-0 is not a rule.