A Toyota Prius plug-in is on display at a conference titled "Plug-in Electric Vehicles: What Role for Washington?" in Washington June 11, 2008. The conference is being hosted by the Brookings Institution and Google.org to place a spotlight on plug-in electric vehicles, examining their potential, their viability, and the pros and cons of different federal policies to promote them. REUTERS/Kevin Lamarque (UNITED STATES)