Facebook has begun rolling out a new feature that uses facial recognition technology to offer suggestions of friends to tag when uploading photos. The feature is turned on by default, it works by comparing new uploads to Facebook's existing archive. Details on how to disable this feature can be found at this FAQ page.
A new privacy option has been made available for users who don't want their faces being recognised, and is enabled by default. The decision to make facial recognition-based tag suggestions opt-out has may be a cause contention for Facebook users who would prefer that any potentially privacy-violating option be opt-in. The concern isn't the feature itself - opting out of automatic tagging doesn't stop friends from tagging you in photos, it only stops Facebook from prompting them to do so - but rather Facebook's 'public by default' attitude.