Well, the camera is able to do facial recognition, consider this an official feature that will be present on the Wii U. With an efficient program, i bet it would not use a lot of resources, for the system to see if you are focusing on the TV or on the padlet.
I can think of several methods, through eye-tracking (easy to spot if your eyes are seeing the DRC or not), face shape-recognition (your head will have different volumes & proportions for the camera depending on what you're watching), etc.
It could be programmed, or even better, calibrated to your physiognomy and setting. I could be in the sofa, the padlet in my belly, me in front of the TV, and i'll teach the software to spot the difference between my head focusing on the main screen (with let's say, a XX° degree angle from the camera) or the padlet.
And then, developers choose which type of graphical sacrifices for the screen that you're not watching, like intensive blur, low details mode, etc.
