I think some models are considering it.

One proposal that's gone out is that for plug-in hybrid cars, which are normally plugged into the grid for charging as they try to use as much grid power as they can, and only burn gas when out of grid power, this would make even more sense.

The reason is the car could sit in the driveway, plugged into the grid. When the grid faced a shortage (hot summer day with lots of AC causing brownouts) it could send a data signal to the cars to fire up and provide power back to the grid. Since peak capacity is the real cost of the power generation system, this could be a win -- though these generators are not nearly as efficient as big ones, and thus would make more pollution. And it had better not fire up in a garage.

But this requires a fancier hookup.