I would say the constraint is that some electronic devices/appliances have information displays or internal programming that need a constant flow of electricity. Your television may lose its channel memory if the power is shut off completely. Any appliance that shows the time or relies on a timer, such as a microwave oven or an electric clock radio, has the clock reset when the power is shut completely off. Devices that use a remote control must have some power on (standby mode) so that you can use the remote to turn them on.

Currently the approach seems to be the design of power strips or sensors that turn off a group of devices. So the power is not being cut off at the appliance end but at the "outlet" or "mains" end. Wikipedia has a good article "Standby Power" which explains all this.

http://en.wikipedia.org/wiki/Standby_power

I am not an engineer, but I think a way to prevent phantom power usage could be designing appliances that use the same principles of hybrid vehicles or laptop computers or mobile phones with batteries. In other words, 2 sources of power, AC and battery. The primary source of power would be AC, but when the device is shut off (into standby mode) it relies on battery power. The battery would be recharging whenever the device is switched on again (in AC mode).