According to an article in Auto Express, the next Prius, to be introduced in 2008, has a target fuel economy of 113 mpg (40km/liter) and will use lighter and smaller lithium-ion batteries. Performance will also be improved by reducing the car's 0-60 mph time by more than a second. The article stated that the motivation was to try to improve European sales, where the car has not been a sucess due to the popularity of diesel models.
The article also noted that both Subaru and Mitsubish both plan to sell electric cars by 2010.
About time!! This is the first mention, that I have seen, of a production car planning to use lithium-ion batteries.
Prius hits 113mpg, Auto Express (UK), April 4, 2006