Philips has launched a monitor with a built-in infrared presence sensor. The Philips Brilliance LCD monitor contains "PowerSensor" technology, which can detect whether you're sat in front of it or not and adjust brightness accordingly.
If you get up and move away from the monitor, then the brightness will immediately dim and reduce power consumption by 50%. All you have to do on your return is sit down again, and it'll realize you're there once more.
Sensitivity is configurable, and the sensor can detect between 30cm and 120cm from the screen. It operates independently of the PC and OS, so it'll work with anything you plug into it. It'll be available in July and cost £170.