Very nice code! I'm so glad I don't have to compile the entire OpenCV library.
However, I'm having the same problem Chun-Yi is having, any other cascade except the included "frontface_alt" gives no results.
Furthermore, detection rates drop very fast at lower resolutions. I actually have to *upsample* my images (to something like 2000x1000 pixels, for group portraits) to detect faces. The "Scale" parameter is supposed to scale images down to 384 pixels, but I can't imagine getting *any* detections at that scale.