I had not seen this paper about automatic face replacement to deal with privacy issues for example.
[...] we present a complete system for automatic face replacement in images. Our system uses a large library of face images created automatically by downloading images from the internet, extracting faces using face detection software, and aligning each extracted face to a common coordinate system.
This is awesome: of course I am thinking about Google street view (or any other mapping system) being entirely populated with people whose faces would be derived from a database of say 50,000 available faces to deal with any privacy issue. That would be interesting. The 50,000 sample faces themselves could be generated from a series of faces! This paper makes for a great read.