The Sony BRAVIA 40 inch WE5 Series HD LCD TV comes with a "Presence Sensor". This can be switched on so that the unit will turn off the screen to save power if it detected no one is in the room. It switches the picture back on instantly when someone comes in. The feature does save power with my measurements showing a drop from 109 Watt to 60 Watt.
Presumably this feature works using an infra-red sensor, as used in low cost burglar alarms. The sensor appears to have a range of about 3.5 m over 120 degrees. The time can be set to 7 seconds to test the feature. It might be set to five minutes for a screen which is only used occasionally or thirty minutes more typically.
The system works very well and would be useful in education and for digital signage. One problem is that the feature does not seem to be active when the TV is receiving VGA input. Otherwise it could be used in meeting rooms and parts of libraries not in constant use. One feature I would like to see is the screen brighten slowly, rather than come on suddenly at full brightness.
Also it would be useful if the feature had a second stage which would switch the unit to standby after a further period of inactivity. That would save more power (dropping consumption to 25 Watt), at the cost of the unit taking longer to restart.
ps: Sony TVs are also available via Amazon.com but may not be versions suitable for Australian TV: Sony BRAVIA W-Series 40-Inch LCD HDTV