An article from the Christian Science Monitor talks about a company in California (among others) that is working on a proposal to allow individuals to sell excess generated power back to the grid from their cars.

Called V2G (Vehicle to Grid), the mechanism is similar to what people who have private power production can already do to sell back their excess power to the grid.

An idea whose time has not yet arrived, it does bring to mind (as described in the article) ideas like fuel cell trucks that can provide temporary power to areas that are experiencing a power outage due to downed power lines.