If you don't want to use mercurial or go to the source code link, here is a zip of the solution that contains a beta release of this library, along with a sample windows phone 7 project that uses the library.
This release should now run on all phones and in all locales. This release includes an optimization in the sample project (MainPage.xaml.cs) which reduces face detection latency from 200ms to 100ms. The trick is to pre-process data (grayscale, histogram equalization) on the downsampled image.
The release also includes the ability to detect multiple faces.
This release also includes a nice 'cameraviewer' control that you can include in other projects. It's a user control that shows you what the camera sees and makes it easy to take pictures with the camera.
This release is now in the 'stable' phase, please report any bugs as they come up on the discussion boards.