With the recent takeover of Face.com by Facebook, programmers have had to rethink how to do face detection tasks. If you don't want to trust a web-based API, what about running one on a phone? Microsoft has the SDK you are looking for.
Microsoft Research seems to just get on with the job and doesn't take its marching orders from marketing. In this case the new SDK that it has just made available for download runs under Windows Phone 7.5 as a managed system. You can use Windows 7 and Visual Studio 2010 to develop applications targeting Windows Phone 7.5 - so there isn't a hint of Windows 8 or Windows Phone 8 in sight.
Currently the SDK is only in beta, but the licence agreement seems to allow you to use it in your own apps whether they are free or paid for. Being managed code you can write apps using C# and basically it's very, very easy. All you have to do is get an image, create a face detection object and let it work on the image to return the location of the faces. You can go a bit deeper if you want to, but only if you like tinkering with the parameters of the algorithm. All of the features provided by the SDK are this easy to use and you don't have to know anything about AI, image processing or rocket science - unless you plan to launch your Windows Phone into space that is.
The SDK offers some interesting functions. You can detect faces across an entire scene and it is supposed to be particularly good at different lighting conditions. As well as detecting faces, it also will provide alignment information for the eyes, brows, mouth, nose etc. so that you can work out the orientation of the face.The detection algorithm is fast enough to be used as a face tracker in a video stream and it can pick up head gestures such as turning left/right to act as input to the phone, i.e. as a sort of software Kinect.
An unusual extra option is is the ability ot create cartoons from the detected face data. You can apply styles and templates to customize the output. One SDK user has already created a cartoon app "Cartoon Me" which you can download for free from the Windows Phone store.
Currently there isn't a version that will work on Windows 7, Windows 8 or Windows Phone 8 but the research team says it is thinking about how to move it to other environments.
Now all we need is for Windows Phone to acquire some users so that we can detect their faces.