A number of jurisdictions have issued guidance on the ethics of looking at metadata. These include New York, Florida, Alabama, Arizona, the District of Columbia, Maryland, and Pennsylvania.
For a summary of the current thinking about metadata, see "Where Do the Footprints of Metadata Lead? Document data mining vexes lawyers and state bar associations," by Marcia Coyle at Law.com.
Links to most of my prior posts on metadata can be found at Legal Underground at the end of the post "New ABA Ethics Opinion on Metadata."