According to ZDNet VeryPC is a company dedicated to making green energy efficient PC's and recently announced that they have released a program called PecoBoo that could help you save monitor energy using your webcam. It detects if you're there via it's facial recognition software and if you're not, it switches the monitor off.
Checking out the software's official website reveals you need to register with your e-mail and phone number or give them a call. Looks like the rest of us will have to wait. Still it looks interesting.