Google email scan helps police arrest a sex offender [Engadget]

Also see Why Google scans your emails for child porn (The Telegraph)

"The bust is notable in that there were no public clues to the offender's activity, such as a website; besides the people he contacted, only Google would have known what he was doing online. That will undoubtedly raise concerns for some, since it wasn't immediately apparent that Mountain View's servers were checking Gmail images. However, the activity isn't a complete surprise. Google's terms of service already indicate that the company is analyzing Gmail for both targeted ads and security -- while illegal pornography isn't explicitly mentioned in the terms, it only makes sense that this content would be considered as well."

Google email scan helps police arrest a sex offender