I should investigate progress in simple face detection methods. These could be the basis of a simple head-switch.
Fast and efficient algorithm for face detection in colour images
The facedetect demo in OpenCV looks useful.
CamTrack has similar goals, using the mean-shift algorithm. Thus page on CamShift looks useful. This one too. Object Tracking Using CamShift Algorithm and Multiple Quantized Feature Spaces Head-tracking pointer at IBM looks interesting.