Some time back Guy Watson wrote an article in the developer center called Webcam Motion Detection: Using the BitmapData API in Flash 8. He later made a better version of it and included that source in the zip file. I went through that code and completely rewrote it in ActionScript 3. Here is the demo. This one of course requires a webcam and flash 9 player to play around.
Once I clean up the code I will post the source as well 🙂