So far the Electric Vehicle is unidirectional: you plug it, then you charge it. For a real Vehicle-to-Home application, the electric cars should be able also to supply the house, discharging the battery on demand.
This is precisely what Honda wants to do with its Power Exporter 9000. In justice, their idea is for the moment not to install it in EV vehicles, but into their fuel cell powered vehicles from 2016.
How useful would it be in practice? According with Honda, it could provide 9kW of electricity (more than enough for powering a house) for a whole week. This could be a very interesting application to run in isolated places where the grid sometimes cannot cope with the demand of electricity.