Arising in the nature of inadvertant attorney client privilege, the question of meta data was recently discussed by Hinshaw:

"The D.C. Bar Association Legal Ethics Committee concluded that when a lawyer has actual knowledge that an adversary has inadvertently provided metadata in an electronic document, the lawyer should not review the metadata without contacting the sending lawyer and abiding by the sender’s instruction. This gives the sender the opportunity to determine if the metadata includes work product or confidential information of the sender’s client. In all other circumstances, however, the receiving lawyer is free to review the metadata contained in electronic files provided by an adversary"