You won't be able to cross the country in a solar powered Prius, but this type of aftermarket hack is exactly the kind of thing so many of our readers have been asking for. Solar Electrical Vehicles says their solar panel modifications (made from mono-crystalline photovoltaic cells) for 2004-2006 Prius models will generate 215 watts of renewable energy that charges a 3kW supplemental battery pack that "provides up to 20 miles per day of electric mode driving range and increased fuel economy." SEV says their system qualifies for up to $2,000 worth of Federal renewable energy tax credits which will then make the whole system pay for itself in two or three years. A solar roof for your Prius will cost between $2,000 and $4,000.
[Source: Solar Electrical Vehicles via Treehugger]