Apple just applied for a patent that would grant it the rights to a zero-power PSU that would use no power when a connected electronic device is turned off.
Apple's idea is based on creating a much more complex AC/DC unit, which consists of the usual regulating circuitry as well as a control circuitry with an attached microprocessor and battery. The control circuitry can determine whether a electronic device is on or off simply by sending data through extra wiring and waiting for a response. If there is a response, the power regulating circuitry is enabled - or it is taken offline if there is no response.
The control circuitry is powered by a battery, which is charged during regular power flow. The patent was filed on April 29, 2011 and there is no indication when it could make it into an actual product. The financial benefit is unclear and the cost to build such a PSU may be higher than the actual power savings in the end. However, there is an environmental benefit, especially if all those millions of devices that Apple sells every quarter use less power.