Well, this sounds like a nice idea, and when i say sound, i really mean it. This new technology comes from Microsoft and it is called "SoundWave". This is how it works in short:
The speakers of the computer are used to emit ultrasound waves, while the microphone is used to sense them back. Thanks to the Doppler effect, the computer can understand whether an obstacle (and more specifically a hand) is moving towards or away from the sound source. With normal sonar technology the distance can be calculated as well.
Though it seems that the page scrolling and other functions are rather... tiring, it still is amazing as a proof of concept. The same technology can be used as a proximity sensor to lock the PC when the person walks away. Additionally, the ultrasound waves can be emitted and sensed even if music is played on the background by the same speakers. Amazing.
As a bottom line, i really do not like it as it is...