I've been thinking of doing this when my i-Miev batteries need replacing. Seems an obvious thing to do if they go down to 80% capacity and are not needed to be traded in on the new pack. So they'd still be capable of holding about 12kWh - enough for my house for 2 days - apart from re-charging the car.
BMW are now offering this for their i3:
I guess the imiev pack will need to be re-arranged to get a suitable output voltage (for charging and running an inverter) ?