New York

The New York County Lawyers'  Association issued this ethics opinion on mining an adversary's responding documents in discovery for metadata (see opinion).  Here's a press release on the opinion.  The New York State Bar Association issued an opinion on the same subject several years ago (see opinion).

I agree with the view that an adversary should not mine a document for metadata; I compare it to the disclosing party inadvertently handing over a privileged document.  If it's obvious that the disclosure is inadvertent, professional courtesy (especially in today's practice of large volume of e-mails and faxes) and ethical considerations warrant returning the privilege document (and ceasing analyzing the document once the inadvertent disclosure becomes clear).

Hat Tip to Sui Generis and Legalethics.com.