Presumably, you'd give quite high incentives to doing this. If someone only charges their car when the grid has excess power, and allows up to 50% of that power to go back into the grid at peak usage, that electricity could even be free. There'd likely be a setting on your charging station to handle this.
You don't need many cars generating 5kW before you're talking about quite a serious amount of power.
Of course, there are problems with this approach - most people park their cars somewhere else during the day, the accounting is an issue, and it seems to be more suited to wind than solar, but I really like the idea of repurposing the massive batteries that we're going to have laying around anyway.
For reference, a Tesla battery pack holds 53kWh [1], which will supply an average home with power for two days [2]. So the numbers really do add up for using it to help during spikes.
[1]: http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system
[2]: http://www.eia.doe.gov/cneaf/electricity/esr/table5.html