August was not just a good month for the Volt or the Leaf but also for sales of the Prius plug-in models. Sales were 1,047 cars, up 52% from July. http://www.greencarcongress.com/2012/09/augustpevsales-20120905.html.
More than 15,000 plug-in Prius models have been sold globally as of August. US sales were 6,068 of that total or about 40% of the world market.
Gasoline pricing is impacting US consumer interest in plug-ins, with August seeing escalating fuel prices, and more plug-in sales.