Using Facelight is a piece of cake. You only need two XAML image controls placed in the same position. The upper one (named ImgResult) will contain a red ellipse specifying the recognized face and the other one (named ImgOverlay) will contain the original bitmap source.
The software is actually a skin recogniser, and can only detect one face per image; its utility is currently limited. It is, however, an excellent example of how Silverlight is helping to bring “write once, run everywhere”, long the holy grail of software developers, closer to reality.
You can read all about Facelight, and download the source code, at Vangos Pterneas’ blog here.